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The Embryonic Period (The First 8 Weeks)

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Table of Contents

  • THE EMBRYONIC PERIOD (THE FIRST 8 WEEKS)
  • THE FETAL PERIOD (8 WEEKS THROUGH BIRTH)
    • Chapter 37 - 9 Weeks: Swallows, Sighs, and Stretches
    • Chapter 38 - 10 Weeks: Rolls Eyes and Yawns, Fingernails & Fingerprints
    • Chapter 39 - 11 Weeks: Absorbs Glucose and Water
    • Chapter 40 - 3 to 4 Months (12 to 16 Weeks): Taste Buds, Jaw Motion, Rooting Reflex, Quickening
    • Chapter 41 - 4 to 5 Months (16 to 20 Weeks): Stress Response, Vernix Caseosa, Circadian Rhythms
    • Chapter 42 - 5 to 6 Months (20 to 24 Weeks): Responds to Sound; Hair and skin; Age of Viability
    • Chapter 43 - 6 to 7 Months (24 to 28 Weeks): Blink-Startle; Pupils response to light; Smell and Taste
    • Chapter 44 - 7 to 8 Months (28 to 32 Weeks): Sound Discrimination, Behavioral States
    • Chapter 45 - 8 to 9 Months (32 to 36 Weeks): Alveoli Formation, Firm Grasp, Taste Preferences
    • Chapter 46 - 9 Months to Birth (36 Weeks through Birth)
    •  
  • APPENDIX A - CALCULATIONS
  • APPENDIX B - RELATING EMBRYONIC AGE & STAGE
  • Bibliography
  • Full Names of Journals Cited
  • Program Index


All embryonic and fetal ages in this program refer to the time since fertilization.
Ages from 4 through 8 weeks are estimated to ± 3 days.
Ages from 8 through 12 weeks are estimated to ± 5 days.
Ages from 12 weeks through birth are generally estimated to ± 1 week.
To simplify age calculations, the term “month“ assumes a 4-week period.
Age and stage conventions adopted during the embryonic period are listed in Appendix B.


English / Deutsch [German]



Chapter 1 Introduction

The dynamic process by which the single-cell human zygote(zī΄gōt)[1] becomes a 100 trillion (1014) cell adult[2] is perhaps the most remarkable phenomenon in all of nature.

Researchers now know that many of the routine functions performed by the adult body become established during pregnancy - often long before birth.[3]

The developmental period before Birth is increasingly understood as a time of preparation during which the developing human acquires the many structures, and practices the many skills needed for survival nach birth.

Chapter 1 Introduction

The dynamic process by which the unicellular human zygote becomes an adult of 100 trillion cells is arguably the most remarkable phenomenon in nature as a whole.

Researchers now know that many of the normal functions of the adult body are determined during pregnancy - often well before birth.

The pre-birth developmental phase is increasingly being understood as a period of preparation during which the developing person acquires the many structures and practices, the many skills, necessary for survival after birth.

Chapter 2 Terminology

Pregnancy in humans normally lasts approximately 38 weeks[4] as measured from the time of fertilization,[5] or conception,[6] until birth.

During the first 8 weeks following fertilization, the developing human is called an embryo,[7] which means "growing within."[8] This time, called the embryonic period,[9] is characterized by the formation of most major body systems.[10]

From the completion of 8 weeks until the end of pregnancy, "the developing human is called a fetus, "which means" unborn offspring. "During this time, called the fetal period, the body grows larger and its systems begin to function.[11]

All embryonic and fetal ages in this program refer to the time since fertilization.[12]

Chapter 2 Terminology

Human pregnancy lasts about 38 weeks, measured from the time of conception or conception to birth.

During the first 8 weeks after fertilization, the developing human being is called an embryo, which means "growing inside". This period, the embryonic phase, is characterized by the formation of most of the bodily systems.

After 8 weeks until the end of pregnancy, “the developing human being is called a fetus,” which means “unborn offspring.” During this time, the fetal phase, the body grows and its systems begin to function.

All embryonic and fetal ages in this program refer to the time since fertilization.

 

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[1]Gasser, 1975, 1.
[2]Guyton and Hall, 2000, 2; Lodish et al., 2000, 12.
[3]Vindla and James, 1995, 598.
[4]Cunningham et al., 2001, 226; O’Rahilly and Müller, 2001, 92.
[5]O’Rahilly and Müller, 1987, 9.
[6]Spraycar, 1995, 377 & 637.
[7]O’Rahilly and Müller, 2001, 87.
[8] Quote from Ayto, 1990, 199.
[9] Human development during the 8-week embryonic period has been divided into a series of 23 stages called Carnegie Stages. These stages are well described in O'Rahilly and Müller, 1987. Because human growth is unique and dependent on multiple factors, different embryos may reach a certain developmental milestone or a certain size at slightly different ages. This internationally-accepted staging system provides a way to describe development independent of age and size. Each of the 23 Carnegie Stages has specific structural features. As we describe various milestones of development, the Carnegie Stage at which they will occur will be noted by a designation such as: [Carnegie Stage 2]. See Appendix B for additional information relating embryonic staging and age assignments.
[10]Moore and Persaud, 2003, 3.
[11] Quotes from Moore and Persaud, 2003, 3: "After the embryonic period (eight weeks), the developing human is called a fetus." Also see O’Rahilly and Müller, 2001, 87.
[12] This convention, termed "postfertilization age" by O'Rahilly, has been long preferred by embryologists. [see Mall, 1918, 400; O’Rahilly and Müller, 1999b, 39; O’Rahilly and Müller, 2001, 88 & 91.] Obstetricians and radiologists typically assign age based on the time elapsed since the first day of the last menstrual period prior to fertilization. This is correctly termed "postmenstrual age" and begins 2 weeks before fertilization occurs. To summarize: postmenstrual age = postfertilization age + 2 weeks. Therefore, postmenstrual age equals approximately 2 weeks at the time of fertilization. The commonly used term "gestational age" has been used with both age conventions and is best either avoided or carefully defined with each use.

Page 3


Embryonic Development: The First 4 Weeks

Chapter 3 Fertilization

Biologically speaking, "human development begins at fertilization,"[13] when a woman and a man each combine 23 of their own chromosomes through the union of their reproductive cells.

A woman's reproductive cell is commonly called an "egg" but the correct term is oocyte (ō´ō-sīt).[14]

Likewise, a man's reproductive cell is widely known as a "sperm," but the preferred term is spermatozoon (sper´mă-tō-zō´on).[15]

Following the release of an oocyte from a woman's ovary in a process called ovulation (ov´yū-lā´shŭn),[16] the oocyte and spermatozoon join within one of the uterine tubes,[17] which are often referred to as Fallopian tubes.

The uterine tubes link a woman's ovaries to her uterus or womb.

The resulting single-celled embryo is called a zygote,[18] meaning "yoked or joined together."[19]

Embryonic Development: The First 4 Weeks

Chapter 3 Fertilization

In biological terms, "human development begins with fertilization" when a woman and a man each combine 23 of their own chromosomes through the union of their reproductive cells.

Female reproductive cells are commonly called "egg cells," but the correct term is oocytes.

Similarly, male reproductive cells are commonly known as "sperm", but the preferred term is spermatozoa.

When an oocyte is rejected from the woman's ovary in a process called ovulation, the oocyte and sperm unite in one of the fallopian tubes, also known as tubes or mother's trumpets.

The fallopian tubes connect a woman's ovaries to her uterus or uterus.

The resulting unicellular embryo is called a zygote, which means something like "joined together" or "connected".

Chapter 4 DNA, Cell Division, and Early Pregnancy Factor (EPF)

DNA

The zygote's 46 chromosomes[20] represent the unique first edition of a new individual's complete genetic blueprint. This master plan resides in tightly coiled molecules called DNA. They contain the instructions for the development of the entire body.

DNA molecules resemble a twisted ladder known as a double helix.[21] The rungs of the ladder are made up of paired molecules, or bases, called guanine, cytosine, adenine, and thymine.

Guanine pairs only with cytosine, and adenine with thymine.[22] Each human cell contains approximately 3 billion (3 × 109) base pairs.[23]

The DNA of a single cell contains so much information that if it were represented in printed words, simply listing the first letter of each base would require over 1.5 million (1.5 × 106) pages of text![24]

If laid end-to-end, the DNA in a single human cell measures 3 feet or 1 meter.[25]

If we could uncoil all of the DNA within an adult's 100 trillion (1014) cells, it would extend over 63 billion (6.3 × 1010) miles. This distance reaches from the earth to the sun and back 340 times.[26]

Cell Division

Approximately 24 to 30 hours after fertilization, the zygote completes its first cell division.[27] Through the process of mitosis, one cell splits into two, two into four, and so on.[28]

Early Pregnancy Factor (EPF)

As early as 24 to 48 hours after fertilization begins, pregnancy can be confirmed by detecting a hormone called "early pregnancy factor"in the mother's blood.[29]

Chapter 4 DNA, Cell Division, and Early Pregnancy Factor (EPF)

DNA

The 46 chromosomes of the zygote sites represent the unique first edition of the complete genetic fingerprint of the new individual. This blueprint resides in the tightly twisted molecules called DNA. They contain the instructions for the development of the entire body.

DNA molecules resemble a twisted ladder, also known as a double helix. The rungs of the ladder are made up of paired molecules or bases called guanine, cytosine, adenine, and thymine.

Guanine only pairs with cytosine and adenine with thymine. Every human cell contains approximately 3 billion of these base pairs.

A single cell's DNA contains so much information that if it were written down in words that simply listed the first letter of each base, it would add up to over 1.5 million pages of text!

In a row, the DNA of a single human cell measures 3 1/3 feet, or 1 meter.

If we rolled up all of the DNA in an adult's 100 trillion cells, it would span 63 billion miles, a distance from Earth to the Sun and back 340 times.

Cell Division

Around 24 to 30 hours after fertilization, the zygote completes its first cell division; through the process of nucleus division, one cell divides into two, then two into four, and so on.

Early Pregnancy Factor (EPF)

As early as 24 to 48 hours after conception, pregnancy can be detected by detecting a hormone, the "early pregnancy factor", in the mother's blood.

 

 


[13] Quote from Moore and Persaud, 2003, 16; From O'Rahilly and Müller, 1987, 9: “Fertilization is the procession of events that begins when a spermatozoon makes contact with an oocyte or its investments and ends with the intermingling of maternal and paternal chromosomes at metaphase of the first mitotic division of the zygote. "See Carlson, 2004, 3; O’Rahilly and Müller, 2001, 8. [Carnegie Stage 1]
[14]O'Rahilly and Müller, 2001, 25: “The term 'egg' should be discarded from human embryology.“ From O'Rahilly and Müller, 1987, 9: “The term 'egg' is best reserved for a nutritive object frequently seen on the breakfast table. "
[15]O’Rahilly and Müller, 2001, 23-24.
[16]O’Rahilly and Müller, 2001, 30.
[17]Dorland and Bartelmez, 1922, 372; Gasser, 1975, 1; Mall, 1918, 421; O’Rahilly and Müller, 2001, 31.
[18]Gasser, 1975, 1; O’Rahilly and Müller, 2001, 33.
[19] Quote from Saunders, 1970, 1; Spraycar, 1995, 1976.
[20]Guyton and Hall, 2000, 34.
[21]Guyton and Hall, 2000, 24; Watson and Crick, 1953, 737.
[22]Guyton and Hall, 2000, 24; Lodish et al., 2000, 103; Watson and Crick, 1953, 737.
[23]Lodish et al., 2000, 456.
[24] See Appendix A.
[25] See Appendix A; Alberts et al., 1998, 189.
[26] See Appendix A.
[27]Hertig, 1968, 26; Hertig and Rock, 1973, 130; (cited by O'Rahilly and Müller, 1987, 12); Shettles, 1958, 400.
[28]Guyton and Hall, 2000, 34.
[29]Moore and Persaud, 2003, 33 & 60; Morton et al., 1992, 72; Nahhas and Barnea, 1990, 105.

Page 4


Chapter 5 Early Stages (Morula and Blastocyst) and Stem Cells

By 3 to 4 days after fertilization, the dividing cells of the embryo assume a spherical shape and the embryo is called a morula (mōr´ū-lă).[30]

By 4 to 5 days, a cavity forms within this ball of cells and the embryo is then called a blastocyst.[31]

The cells inside the blastocyst are called the inner cell mass and give rise to the head, body, and other structures vital to the developing human.[32]

Cells within the inner cell mass are called embryonic stem cells because they have the ability to form each of the more than 200 cell types contained in the human body.[33]

Chapter 5 Early Stages (Morula and Blastocyst) and Stem Cells

Up to 3 to 4 days after fertilization, the dividing cells of the embryo take on a spherical shape, and the embryo is called a morula.

A cavity forms in this cell ball within 4 to 5 days, and the embryo is called the germinal vesicle.

The cells within the germinal vesicle are called the inner cell mass, from which the head, body and other structures are formed that are important for the developing human being.

Cells in the inner cell mass are called embryonic stem cells because they can make any of the more than 200 types of cells found in the human body.

Chapter 6 1 to 1½ Weeks: Implantation and Human Chorionic Gonadotropin (hCG)

After traveling down the uterine tube, the early embryo embeds itself into the inner wall of the mother's uterus. This process, called implantation, begins 6 days and ends 10 to 12 days after fertilization.[34]

Cells from the growing embryo begin to produce a hormone called human chorionic gonadotropin (human kō-rē-on'ik gō'nad-ō-trō'pin), or hCG, the substance detected by most pregnancy tests.[35]

HCG directs maternal hormones to interrupt the normal menstrual cycle, allowing pregnancy to continue.[36]

Chapter 6 1 to 1½ Weeks: Implantation and Human Chorionic Gonadotropin (hCG)

After passing through the fallopian tube, the early embryo embeds itself in the inner wall of the mother's uterus. This process, implantation, begins 6 days and ends 10 to 12 days after fertilization.

Cells in the growing embryo begin to produce a hormone called human chorionic gonadotropin, or HCG, the substance most pregnancy tests detect.

HCG causes the maternal hormones to interrupt the normal menstrual cycle and allow the pregnancy to continue.

Chapter 7 The Placenta and Umbilical Cord

Following implantation, cells on the periphery of the blastocyst give rise to part of a structure called the placenta (plă-sen'tă), which serves as an interface between the maternal and embryonic circulatory systems.

The placenta delivers maternal oxygen, nutrients, hormones, and medications to the developing human; removes all waste products; and maternal blood from mixing with the blood of the embryo and fetus prevents.[37]

The placenta also produces hormones and maintains embryonic and fetal body temperature slightly above that of the mother's.[38]

The placenta communicates with the developing human through the vessels of the umbilical (ŭm-bil'i-kăl) cord.[39]

The life support capabilities of the placenta rival those of intensive care units found in modern hospitals.

Chapter 7 The Placenta and Umbilical Cord

As a result of the implantation, the cells create a structure in the vicinity of the germinal vesicle, the placenta, which serves as a link between the maternal and embryonic circulatory systems.

The placenta provides the mother's oxygen and nutrients, hormones, and medication to the developing human, clearing all waste, and preventing the mother's blood from mixing with the embryo and fetus blood.

The placenta also produces hormones and maintains the temperature of the embryo and fetus slightly above that of the mother.

The placenta communicates with the developing human through the vessels of the umbilical cord.

The life support measures of the placenta are on a par with those of an intensive care unit in a modern hospital.

 

 


[30]Gasser, 1975, 1; O’Rahilly and Müller, 2001, 37; Spraycar, 1995, 1130: “Morula” is derived from the Latin word morus meaning "mulberry." [Carnegie Stage 2]
[31]O’Rahilly and Müller, 2001, 39. [Carnegie Stage 3]
[32]Gasser, 1975, 1; O’Rahilly and Müller, 2001, 39; Sadler, 2005, 6.
[33]Alberts et al., 1998, 32. For a discussion and definition of embryonic stem cells see the website of the National Institutes of Health: http://stemcells.nih.gov/infoCenter/stemCellBasics.asp#3
[34]O’Rahilly and Müller, 2001, 40; Implantation begins with attachment of the blastocyst at about 6 days after fertilization. [Attachment of the blastocyst to the inner wall of the uterus is a transient event and is the hallmark of Carnegie Stage 4.] See also Adams, 1960, 13-14; Cunningham et al., 2001, 20; Hamilton, 1949, 285-286; Hertig, 1968, 41; Hertig and Rock, 1944, 182; Hertig and Rock, 1945, 81 & 83; Hertig and Rock, 1949, 183; Hertig et al., 1956, 444. [Carnegie Stage 5]
[35]Chartier et al., 1979, 134; Cunningham et al., 2001, 27; O’Rahilly and Müller, 2001, 43.
[36]Cunningham et al., 2001, 20 & 26-27; O’Rahilly and Müller, 2001, 31.
[37]Hertig, 1968, 16; Cunningham et al., 2001, 86 & 136; For a detailed description of the placenta see Hamilton and Boyd, 1960. For a detailed description of the placenta vasculature see Harris and Ramsey, 1966. This separation of maternal and fetal blood is almost but not quite perfect as a small number of fetal cells may be found in the maternal circulation and vice-versa. See Cunningham et al., 2001, 96 & 136.
[38]Liley, 1972, 101; O’Rahilly and Müller, 2001, 78-79.
[39] For a detailed description of umbilical cord formation see Florian, 1930.

Page 5


Chapter 8 Nutrition and Protection

By 1 week, cells of the inner cell mass form two layers called the hypoblast other epiblast.[40]

The hypoblast gives rise to the yolk sac,[41] which is one of the structures through which the mother supplies nutrients to the early embryo.[42]

Cells from the epiblast form a membrane called the amnion (am-nē-on),[43] within which the embryo and later the fetus develop until birth.

Chapter 8 Nutrition and Protection

Up to 1 week, the cells of the inner cell mass form two layers, hypoblast and epiblast.

The hypoblast creates the yolk sac, one of the structures through which the mother supplies nutrients to the embryo in its early stages.

Cells from the epiblast form a membrane, the embryonic envelope, in which the embryo and later the fetus develop until birth.

Chapter 9 2 to 4 Weeks: Germ Layers and Organ Formation

By approximately 2½ weeks, the epiblast has formed 3 specialized tissues, or germ layers, called ectoderm, endoderm, other mesoderm.[44]

Ectoderm gives rise to numerous structures including the brain, spinal cord, nerves, skin, nails, and hair.

Endoderm produces the lining of the respiratory system and digestive tract and generates portions of major organs such as the liver and pancreas.

Mesoderm forms the heart, kidneys, bones, cartilage, muscles, blood cells, and other structures.[45]

By 3 weeks the brain is dividing into 3 primary sections called the forebrain, midbrain, and hindbrain.[46]

Development of the respiratory and digestive systems is also underway.[47]

As the first blood cells appear in the yolk sac,[48] blood vessels form throughout the embryo, and the tubular heart emerges.[49]

Almost immediately, the rapidly growing heart folds in upon itself as separate chambers begin to develop.[50]

The heart begins beating 3 weeks and 1 day following fertilization.[51]

The circulatory system is the first body system, or group of related organs, to achieve a functional state.[52]

Chapter 9 2 to 4 Weeks: Germ Layers and Organ Formation

After about 2 1/2 weeks, the epiblast3 has formed specialized tissues or cotyledons, the ectoderm, the endoderm, and the mesoderm.

The ectoderm forms numerous structures including the brain, spinal cord, nerves, skin, fingernails, and hair.

The endoderm forms the covering of the respiratory and digestive tracts, and creates parts of important organs such as the liver and pancreas.

The mesoderm makes up the heart, kidneys, bones, cartilage, muscles, blood cells, and other structures.

After 3 weeks the brain divides into 3 main regions, forebrain, midbrain, and posterior brain.

The development of the respiratory and digestive tract is also ongoing.

When the first blood cells appear in the yolk sac, blood vessels form within the embryo and the tubular heart is formed.

Almost immediately, the growing heart folds in on itself as the separate heart chambers begin to develop.

The heart starts beating 3 weeks and 1 day after fertilization.

The circulatory system is the first body system, the first group of related organs to reach a functional stage.

Chapter 10 3 to 4 Weeks: The Folding of the Embryo

Between 3 and 4 weeks, the body plan emerges as the brain, spinal cord, and heart of the embryo are easily identified alongside the yolk sac.

Rapid growth causes folding of the relatively flat embryo.[53] This process incorporates part of the yolk sac into the lining of the digestive system and forms the chest and abdominal cavities of the developing human.[54]

Chapter 10 3 to 4 Weeks: The Folding of the Embryo

Between 3 and 4 weeks the body plan appears, and the embryo's brain, spinal cord, and heart are easily identified next to the yolk sac.

Rapid growth leads to a folding of the relatively flat embryo. This process encloses a portion of the yolk sac within the envelope of the digestive system and forms the thoracic and abdominal cavities of the developing human.

 

 


[40]O’Rahilly and Müller, 2001, 39.
[41]Moore and Persaud, 2003, 50; O'Rahilly and Müller, 2001, 82. [Carnegie Stages 5 & 6]; In humans, the term "yolk sac" has fallen out of favor among some embryologists (including O’Rahilly and Müller) because it is not a nutrient reservoir and does not contain yolk. The technically preferred term is umbilical vesicle. This structure plays a vital role in the transfer of nutrients from mother to embryo before placental circulation becomes fully functional.
[42]Campbell et al., 1993, 756; Kurjak et al., 1994, 437; O’Rahilly and Müller, 2001, 82.
[43]O'Rahilly and Müller, 1987, 29; O’Rahilly and Müller, 2001, 43. [Carnegie Stages 4-5]
[44]O’Rahilly and Müller, 2001, 14 & 135. [Carnegie Stage 7]; It should be noted there are many examples of organs derived from multiple germ layers. For instance, the liver is largely derived from endoderm but contains blood vessels and blood cells derived from mesoderm and nerves of ectodermal origin.
[45]Moore and Persaud, 2003, 80 & 83; Sadler, 2005, 9.
[46]Bartelmez, 1923, 236; Müller and O'Rahilly, 1983, 419-420 & 429; O’Rahilly and Gardner, 1979, 123 & 129; O'Rahilly and Müller, 1984, 422; O'Rahilly and Müller, 1987, 90; O’Rahilly and Müller, 1999a, 47 & 52. [Carnegie Stage 9]
[47]DiFiore and Wilson, 1994, 221; Fowler et al., 1988, 793; Grand et al., 1976, 793-794 & 796 & 798; O'Rahilly, 1978, 125; O’Rahilly and Boyden, 1973, 238-239; O'Rahilly and Müller, 1984, 421; O’Rahilly and Tucker, 1973, 6 & 8 & 23; Streeter, 1942, 232 & 235.
[48]Carlson, 2004, 117.
[49]Gilmour, 1941, 28; O'Rahilly and Müller, 1987, 86. [Carnegie Stage 9]
[50]Campbell, 2004, 14; Carlson, 2004, 116 & 446; Navaratnam, 1991, 147-148; O'Rahilly and Müller, 1987, 99. [Carnegie Stage 10]
[51]Campbell, 2004, 14; Carlson, 2004, 430; De Vries and Saunders, 1962, 96; Gardner and O'Rahilly, 1976, 583; Gilbert-Barness and Debich-Spicer, 1997, 650; Gittenger-de Groot et al., 2000, 17; van Heeswijk et al., 1990, 151; Kurjak and Chervenak, 1994, 439; Navaratnam, 1991, 147-148; O'Rahilly and Müller, 1987, 99; Wisser and Dirschedl, 1994, 108. [Carnegie Stage 10, possibly late Stage 9]
[52]Moore and Persaud, 2003, 70: "The cardiovascular system is the first organ system to reach a functional state."
[53]Moore and Persaud, 2003, 78.
[54]Gasser, 1975, 26; Moore and Persaud, 2003, 78.

Page 6


Embryonic Development: 4 to 6 Weeks

Chapter 11 4 Weeks: Amniotic Fluid

By 4 weeks the clear amnion surrounds the embryo in a fluid-filled sac.[55] This sterile liquid, called amniotic (am-nē-ot'ik) fluid, provides the embryo with protection from injury.[56]

Embryonic Development: 4 to 6 Weeks

Chapter 11 4 Weeks: Amniotic Fluid

For up to 4 weeks, the clear amniotic fluid surrounds the embryo in a bag filled with fluid. This sterile fluid, the amniotic fluid, protects the embryo from injury.

Chapter 12 The Heart in Action

The heart typically beats about 113 times per minute.[57]

Note how the heart changes color as blood enters and leaves its chambers with each beat.

The heart will beat approximately 54 million (5.4 × 107) times before birth and over 3.2 billion (3.2 × 109) times over the course of an 80-year lifespan.[58]

Chapter 12 The Heart in Action

The heart usually beats about 113 times per minute.

Notice how the heart changes color as the blood enters and exits the chambers with each heartbeat.

The heart beats about 54 million times before birth and over 3.2 billion times during an 80-year life.

Chapter 13 Brain Growth

Rapid brain growth is evidenced by the changing appearance of the forebrain, midbrain, and hindbrain.

Chapter 13 Brain Growth

Rapid brain growth is shown by changes in the appearance of the forebrain, midbrain, and posterior brain.

Chapter 14 Limb Buds

Upper and lower limb development begins with the appearance of the limb buds by 4 weeks.[59]

The skin is transparent at this point because it is only one cell thick.

As the skin thickens, it will lose this transparency, which means that we will only be able to watch internal organs develop for about another month.[60]

Chapter 14 Limb Buds

The development of the lower extremities begins with the emergence of extremity buds for up to 4 weeks.

The skin is transparent at this point because it is only one cell thick.

As the skin thickens, it loses its transparency. we can only follow the development of the internal organs for about a month.

Chapter 15 5 Weeks: Cerebral Hemispheres

Between 4 and 5 weeks, the brain continues its rapid growth and divides into five distinct sections.[61]

The head comprises about one-third of the embryo's total size.[62]

The cerebral (ser'ĕ-brăl) hemispheres appear,[63] gradually becoming the largest parts of the brain.[64]

Functions eventually controlled by the cerebral hemispheres include thought, learning, memory, speech, vision, hearing, voluntary movement, and problem-solving.[65]

Chapter 15 5 Weeks: Cerebral Hemispheres

Between 4 and 5 weeks the brain continues its rapid growth and divides into 5 different sectors.

The head is about 1/3 the total size of the embryo.

The two halves of the brain appear and eventually form the largest part of the brain.

Functions are at some point controlled by the hemispheres, including thinking, learning, memory, language, sight, hearing, intentional movement, and problem solving.

 

 


[55]Gasser, 1975, 30; O’Rahilly and Müller, 2001, 80.
[56]O’Rahilly and Müller, 2001, 81.
[57]van Heeswijk et al., 1990, 153.
[58] See Appendix A.
[59]Gasser, 1975, 49 & 59; O'Rahilly and Gardner, 1975, 11; O’Rahilly and Müller, 1985, 148 & 151; O’Rahilly and Müller, 1987, 143; Streeter, 1945, 30; Uhthoff, 1990, 7 & 141. [upper and lower limb buds: Carnegie Stages 12 & 13]
[60]Moore and Persaud, 2003, 486; O'Rahilly, 1957, 459; O’Rahilly and Müller, 2001, 165. For information about the first-trimester, direct-imaging technique used in this program (called embryoscopy), see Cullen et al., 1990.
[61]O’Rahilly and Müller, 1999a, 134; Sadler, 2005, 106. [Carnegie Stage 15]
[62]Laffont, 1982, 5.
[63]Bartelmez and Dekaban, 1962, 25; Campbell, 2004, 17; O'Rahilly and Gardner, 1979, 130; O'Rahilly et al., 1984, 249; O’Rahilly and Müller, 1999a, 115; van Dongen and Goudie, 1980, 193. [Carnegie Stage 14]
[64]Moore, 1980, 938.
[65]Guyton and Hall, 2000, 663-677.

Page 7


Chapter 16 Major Airways

In the respiratory system, the right and left main stem bronchi (brong'kī) are present[66] and will eventually connect the trachea (trā´kē-ă), or windpipe, with the lungs.

Chapter 16 Major Airways

The right and left main trunk bronchi are present in the respiratory tract and eventually connect to the lungs via the trachea or windpipe.

Chapter 17 Liver and Kidneys

Note the massive liver filling the abdomen adjacent to the beating heart.

The permanent kidneys appear by 5 weeks.[67]

Chapter 17 Liver and Kidneys

Note the massive liver that fills the belly next to the beating heart.

The final kidneys appear by 5 weeks.

Chapter 18 Yolk Sac and Germ Cells

The yolk sac contains early reproductive cells called germ cells. By 5 weeks these germ cells migrate to the reproductive organs adjacent to the kidneys.[68]

Chapter 18 Yolk Sac and Germ Cells

The yolk sac contains reproductive cells in the early stages, the germ cells. These germ cells migrate to the reproductive organs next to the kidneys for up to 5 weeks.

Chapter 19 Hand Plates and Cartilage

Also by 5 weeks, the embryo develops hand plates,[69] and cartilage formation begins by 5½ weeks.[70]

Here we see the left hand plate and wrist at 5 weeks and 6 days.

Chapter 19 Hand Plates and Cartilage

The embryo also develops palms up to 5 weeks and the formation of cartilage tissue begins up to 5 1/2 weeks.

Here we see the left palm and wrist after 5 weeks and 6 days.

 

 


[66]Moore and Persaud, 2003, 245; O’Rahilly and Boyden, 1973, 239; O’Rahilly and Müller, 2001, 291; Sparrow et al., 1999, 550.
[67]Angtuaco et al., 1999, 13; Lipschutz, 1998, 384; Moore and Persaud, 2003, 288; O’Rahilly and Müller, 1987, 167 & 182; O’Rahilly and Müller, 2001, 301; Sadler, 2005, 72. [Carnegie Stage 14]
[68]O’Rahilly and Müller, 2001, 23; Waters and Trainer, 1996, 16; Witschi, 1948, 70, 77 & 79.
[69]O'Rahilly and Müller, 1987, 175; Streeter, 1948, 139. [Carnegie Stage 15]
[70]O'Rahilly and Gardner, 1975, 4. [Carnegie Stages 16 and 17]

Page 8


Embryonic Development: 6 to 8 Weeks

Chapter 20 6 Weeks: Motion and Sensation

By 6 weeks the cerebral hemispheres are growing disproportionately faster than other sections of the brain.

The embryo begins to make spontaneous and reflexive movements.[71] Such movement is necessary to promote normal neuromuscular development.

A touch to the mouth area causes the embryo to reflexively withdraw its head.[72]

Embryonic Development: 6 to 8 Weeks

Chapter 20 6 Weeks: Motion and Sensation

Up to 6 weeks, the halves of the brain grow disproportionately faster than other parts of the brain.

The embryo begins to move spontaneously and reflexively. Such movements are necessary to allow normal neuromuscular development.

Touching the area around the mouth causes the embryo to pull in its head as a reflex.

Chapter 21 The External Ear and Blood Cell Formation

The external ear is beginning to take shape.[73]

By 6 weeks, blood cell formation is underway in the liver where lymphocytes are now present.[74] This type of white blood cell is a key part of the developing immune system.

Chapter 21 The External Ear and Blood Cell Formation

The outer ear begins to form.

For up to 6 weeks, blood cell formation takes place in the liver, where lymphocytes are now present. This type of white blood cell is an essential part of the development of the immune system.

Chapter 22 The Diaphragm and Intestines

The diaphragm (dī'ă-fram), the primary muscle used in breathing, is largely formed by 6 weeks.[75]

A portion of the intestine now protrudes temporarily into the umbilical cord. This normal process, called physiologic herniation (fiz-ē-ō-loj'ik her-nē-ā'shŭn), makes room for other developing organs in the abdomen.[76]

Chapter 22 The Diaphragm and Intestines

The diaphragm, the main muscle used for breathing, is largely developed by 6 weeks.

Part of the intestine now temporarily extends into the umbilical cord. This normal process, called physiological herniation, gives way to the development of other organs in the abdomen.

Chapter 23 Hand Plates and Brainwaves

At 6 weeks the hand plates develop a subtle flattening.[77]

Primitive brainwaves have been recorded as early as 6 weeks and 2 days.[78]

Chapter 23 Hand Plates and Brainwaves

After 6 weeks the palms develop a fine flattening.

Primitive brain waves were already detected after 6 weeks and 2 days.

 

 


[71]Birnholz et al., 1978, 539; de Vries et al., 1982, 301 & 304: “The first movements were observed at 7.5 weeks postmenstrual age.“ [or 5½ weeks postfertilization age]; Humphrey, 1964, 99: earliest reflex 5½ weeks; Humphrey, 1970, 12; Humphrey and Hooker, 1959, 76; Humphrey and Hooker, 1961, 147; Kurjak and Chervenak, 1994, 48; Visser et al., 1992, 175-176: “Endogenously generated fetal movements can first be observed after 7 weeks postmenstrual age (i.e. 5 weeks after conception);“ Natsuyama, 1991, 13; O’Rahilly and Müller, 1999a, 336: 5½ weeks postfertilization; Sorokin and Dierker, 1982, 723 & 726; Visser et al., 1992, 175-176; Natsuyama, 1991, 13: Spontaneous movement observed by “Carnegie stage 15“ (about 33 days postfertilization); Hogg, 1941, 373: Reflex activity begins at 6½ weeks [adjusted to postfertilization age].
[72]Goodlin, 1979, D-128.
[73]Karmody and Annino, 1995, 251; O’Rahilly and Müller, 2001, 480; Streeter, 1948, 190.
[74]Kurjak and Chervenak, 1994, 19.
[75]de Vries et al., 1982, 320.
[76]Gilbert-Barness and Debich-Spicer, 1997, 774; Grand et al., 1976, 798; O'Rahilly and Müller, 1987, 213; Sadler, 2005, 66; Spencer, 1960, 9; Timor-Tritsch et al., 1990, 287.
[77]O’Rahilly and Müller, 1987, 202-203.
[78]Borkowski and Bernstine, 1955, 363 (cited by Bernstine, 1961, 63 & 66; O’Rahilly and Müller, 1999a, 195; van Dongen and Goudie, 1980, 193.); Hamlin, 1964, 113. For a summary of in utero fetal encephalography (measuring brainwaves) in the near-term fetus using abdominal and vaginal electrodes see Bernstine et al., 1955.

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Chapter 24 Nipple Formation

Nipples appear along the sides of the trunk shortly before reaching their final location on the front of the chest.[79]

Chapter 24 Nipple Formation

Nipples appear on the sides of the torso just before they reach their final position on the front of the chest.

Chapter 25 Limb Development

By 6½ weeks, the elbows are distinct, the fingers are beginning to separate,[80] and hand movement can be seen.

Bone formation, called ossification (os'i-fi-kā'shŭn), begins within the clavicle, or collar bone, and the bones of the upper and lower jaw.[81]

Chapter 25 Limb Development

By 6 1/2 weeks, the elbows are formed, the fingers begin to separate, and hand movements are visible.

Bone formation, or ossification, begins with the collarbone, the clavicle, and the bones of the upper and lower jaw.

Chapter 26 7 Weeks: Hiccups and Startle Response

Hiccups have been observed by 7 weeks.[82]

Leg movements can now be seen, along with a startle response.[83]

Chapter 26 7 Weeks: Hiccups and Startle Response

Hiccups were registered for up to 7 weeks.

Leg movements can now be observed, as well as startle movements.

Chapter 27 The Maturing Heart

The four-chambered heart is largely complete.[84] On average, the heart now beats 167 times per minute.[85]

Electrical activity of the heart recorded at 7½ weeks reveals a wave pattern similar to the adult's.[86]

Chapter 27 The Maturing Heart

Most of the four chambers of the heart are complete; the heart now beats an average of 167 times per minute.

The electrical activity of the heart up to 7 1/2 weeks shows a wave motion similar to that of an adult.

Chapter 28 Ovaries and Eyes

In females, the ovaries are identifiable by 7 weeks.[87]

By 7½ weeks, the pigmented retina of the eye is easily seen and the eyelids are beginning a period of rapid growth.[88]

Chapter 28 Ovaries and Eyes

In female embryos, the ovaries can be detected for up to 7 weeks.

The pigmented retinal can easily be seen for up to 7 1/2 weeks and the eyelids begin an intense growth phase.