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Figure 6-3 Diagram illustrating the changing proportions of the body during the fetal period. At 9 weeks, the head is approximately half the crown-heel length of the fetus.
By 36 weeks, the circumferences of the head and the abdomen are approximately equal. After this (38 weeks), the circumference of the abdomen may be greater. All
stages are drawn to the same total height.
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Figure 6-4 A 9-week fetus in the amniotic sac exposed by removal from the chorionic sac. A, Actual size. The remnant of the umbilical vesicle is indicated by an arrow. B,
Enlarged photograph of the fetus (×2). Note the following features: large head, fused eyelids, cartilaginous ribs, and intestines in umbilical cord (arrow). (Courtesy
of Professor Jean Hay [retired], Department of Human Anatomy and Cell Science, University of Manitoba, Winnipeg, Manitoba, Canada.)
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Figure 6-5 An 11-week fetus exposed by removal from its chorionic and amniotic sacs (×1.5). Note its relatively large head and that the intestines are no longer in
the umbilical cord. (Courtesy of Professor Jean Hay [retired], Department of Human Anatomy and Cell Science, University of Manitoba, Winnipeg, Manitoba, Canada.)
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Figure 6-13 Graph showing the rate of fetal growth during the last trimester. Average refers to babies born in the United States. After 36 weeks, the growth rate deviates
from the straight line. The decline, particularly after full term (38 weeks), probably reflects inadequate fetal nutrition caused by placental changes. (Adapted from
Gruenwald P: Growth of the human fetus. I. Normal growth and its variation. Am J Obstet Gynecol 94:1112-1119, 1966.)
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Figure 6-15 A, Illustration of amniocentesis. A needle is inserted through the lower abdominal and uterine walls into the amniotic cavity. A syringe is attached and
amniotic fluid is withdrawn for diagnostic purposes. B, Drawing illustrating chorionic villus sampling. Two sampling approaches are illustrated: through the maternal
anterior abdominal wall with a needle and through the vagina and cervical canal using a malleable catheter.
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Figure 7-1 Development of the placenta and fetal membranes. A, Frontal section of the uterus showing elevation of the decidua capsularis by the expanding chorionic
sac of a 4-week embryo implanted in the endometrium on the posterior wall (*). B, Enlarged drawing of the implantation site. The chorionic villi were exposed by cutting
an opening in the decidua capsularis. C to F, Sagittal sections of the gravid uterus from weeks 5 to 22 showing the changing relations of the fetal membranes to the
decidua. In F, the amnion and chorion are fused with each other and the decidua parietalis, thereby obliterating the uterine cavity. Note in D to F that the chorionic villi
persist only where the chorion is associated with the decidua basalis.
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Figure 7-2 A, Lateral view of a spontaneously aborted embryo at Carnegie stage 14, approximately 32 days. The chorionic and amniotic sacs have been opened to show
the embryo. Note the large size of the umbilical vesicle at this stage. B, The sketch shows the actual size of the embryo and its membranes. (A, From Moore KL, Persaud
TVN, Shiota K: Color Atlas of Clinical Embryology, 2nd ed. Philadelphia, WB Saunders, 2000.)
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Figure 7-4 Spontaneously aborted human chorionic sac containing a 13-week fetus. The smooth chorion formed when the chorionic villi degenerated and disappeared
from this area of the chorionic sac. The villous chorion is where chorionic villi persist and form the fetal part of the placenta. In situ, the cotyledons were attached to the
decidua basalis and the intervillous space was filled with maternal blood. (From Moore KL, Persaud TVN, Shiota K: Color Atlas of Clinical Embryology, 2nd ed.
Philadelphia, WB Saunders, 2000.)
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Figure 7-6 Drawing of a sagittal section of a gravid uterus at 4 weeks shows the relation of the fetal membranes to each other and to the decidua and embryo. The
amnion and smooth chorion have been cut and reflected to show their relationship to each other and the decidua parietalis.
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Figure 7-7 Schematic drawing of a transverse section through a full-term placenta, showing (1) the relation of the villous chorion (fetal part of placenta) to the decidua
basalis (maternal part of placenta), (2) the fetal placental circulation, and (3) the maternal placental circulation. Maternal blood flows into the intervillous space in funnelshaped spurts from the spiral endometrial arteries, and exchanges occur with the fetal blood as the maternal blood flows around the branch villi. It is through these villi
that the main exchange of material between the mother and embryo/fetus occurs. The inflowing arterial blood pushes venous blood out of the intervillous space into the
endometrial veins, which are scattered over the surface of the decidua basalis. Note that the umbilical arteries carry poorly oxygenated fetal blood (shown in blue) to the
placenta and that the umbilical vein carries oxygenated blood (shown in red) to the fetus. Note that the cotyledons are separated from each other by placental septa,
projections of the decidua basalis. Each cotyledon consists of two or more main stem villi and many branch villi. In this drawing, only one stem villus is shown in each
cotyledon, but the stumps of those that have been removed are indicated.
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Figure 7-8 A, Drawing of a stem chorionic villus showing its arteriocapillary-venous system. The arteries carry poorly oxygenated fetal blood and waste products from the
fetus, whereas the vein carries oxygenated blood and nutrients to the fetus. B and C, Drawings of sections through a branch villus at 10 weeks and full term, respectively.
The placental membrane, composed of extrafetal tissues, separates the maternal blood in the intervillous space from the fetal blood in the capillaries in the villi. Note that
the placental membrane becomes very thin at full term. Hofbauer cells are thought to be phagocytic cells.
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Figure 7-9 Diagrammatic illustration of transfer across the placental membrane (barrier). The extrafetal tissues, across which transport of substances between the mother
and fetus occurs, collectively constitute the placental membrane. Inset, Light micrograph of chorionic villus showing a fetal capillary (arrow) and the placental membrane.
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Figure 7-11 Drawings illustrating parturition (childbirth). A and B, The cervix is dilating during the first stage of labor. C to E, The fetus is passing through the cervix and
vagina during the second stage of labor. F and G, As the uterus contracts during the third stage of labor, the placenta folds and pulls away from the uterine wall.
Separation of the placenta results in bleeding and formation of a large hematoma (mass of blood). Pressure on the abdomen facilitates placental separation. H, The
placenta is expelled and the uterus contracts.
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Figure 7-16 Placental abnormalities. In placenta accreta, there is abnormal adherence of the placenta to the myometrium. In placenta percreta, the placenta has
penetrated the full thickness of the myometrium. In this example of placenta previa, the placenta overlies the internal os of the uterus and blocks the cervical canal.
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Figure 7-22 Illustrations showing how the amnion enlarges, obliterates the chorionic cavity, and envelops the umbilical cord. Observe that part of the umbilical vesicle is
incorporated into the embryo as the primordial gut. Formation of the fetal part of the placenta and degeneration of chorionic villi are also shown. A, At 3 weeks; B, at 4
weeks; C, at 10 weeks; D, at 20 weeks.
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Figure 7-24 Illustrations of the development and usual fate of the allantois. A, A 3-week embryo. B, A 9-week fetus. C, A 3-month male fetus. D, Adult female. The
nonfunctional allantois forms the urachus in the fetus and the median umbilical ligament in the adult.
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Figure 7-26 Diagrams illustrating how dizygotic twins develop from two zygotes. The relationships of the fetal membranes and placentas are shown for instances in which
the blastocysts implant separately (A) and the blastocysts implant close together (B). In both cases, there are two amnions and two chorions. The placentas are usually
fused when they implant close together.
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Figure 7-27 Diagrams illustrating how approximately 65% of monozygotic twins develop from one zygote by division of the embryoblast (inner cell mass) of the blastocyst.
These twins always have separate amnions, a single chorionic sac, and a common placenta. If there is anastomosis of the placental vessels, one twin may receive most
of the nutrition from the placenta. Inset, Monozygotic twins, 17 weeks' gestation. (Courtesy of Dr. Robert Jordan, St. Georges University Medical School, Grenada.)
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Figure 7-29 Diagrams illustrating how approximately 35% of monozygotic twins develop from one zygote. Separation of the blastomeres may occur anywhere from the
two-cell stage to the morula stage, producing two identical blastocysts. Each embryo subsequently develops its own amniotic and chorionic sacs. The placentas may be
separate or fused. In 25% of cases, there is a single placenta resulting from secondary fusion, and in 10% of cases, there are two placentas. In the latter cases,
examination of the placenta would suggest that they were dizygotic twins. This explains why some monozygotic twins are wrongly stated to be dizygotic twins at birth.
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Figure 7-30 Diagrams illustrating how some monozygotic twins develop. This method of development is very uncommon. Division of the embryonic disc results in two
embryos within one amniotic sac. A, Complete division of the embryonic disc gives rise to twins. Such twins rarely survive because their umbilical cords are often so
entangled that interruption of the blood supply to the fetuses occurs. B and C, Incomplete division of the disc results in various types of conjoined twins.
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Figure 12-1 A, Dorsal view of an embryo during the third week (approximately 18 days). B, Transverse section of the embryo showing the position of the intermediate
mesenchyme before lateral folding of the embryo. C, Lateral view of an embryo during the fourth week (approximately 24 days). D, Transverse section of the embryo
after the commencement of folding, showing the nephrogenic cords. E, Lateral view of an embryo later in the fourth week (approximately 26 days). F, Transverse section
of the embryo showing the lateral folds meeting each other ventrally. Observe the position of the urogenital ridges and nephrogenic cords.
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Figure 12-2 Illustrations of the three sets of excretory systems in an embryo during the fifth week. A, Lateral view. B, Ventral view. The mesonephric tubules have been
pulled laterally; their normal position is shown in A.
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Figure 12-5 A, Lateral view of a 5-week embryo showing the extent of the early mesonephros and the primordium of the metanephros (primordium of permanent kidney.)
B, Transverse section of the embryo showing the nephrogenic cords from which the mesonephric tubules develop. C to F, Successive stages in the development of a
mesonephric tubule between the 5th and 11th weeks. Note that the mesenchymal cell cluster in the nephrogenic cord develops a lumen, thereby forming a mesonephric
vesicle. The vesicle soon becomes an S-shaped mesonephric tubule and extends laterally to join the mesonephric duct. The expanded medial end of the mesonephric
tubule is invaginated by blood vessels to form a glomerular capsule.
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Figure 12-6 Development of the permanent kidney. A, Lateral view of a 5-week embryo showing the primordium of the metanephros. B to E, Successive stages in the
development of the metanephric diverticulum (fifth to eighth weeks). Observe the development of the ureter, renal pelvis, calices, and collecting tubules.
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Figure 12-7 Development of nephrons. A, Nephrogenesis commences around the beginning of the eighth week. B and C, Note that the metanephric tubules, the
primordia of the nephrons, become continuous with the collecting tubules to form uriniferous tubules. D, Observe that nephrons are derived from the metanephrogenic
blastema and that the collecting tubules are derived from the metanephric diverticulum.
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Figure 12-10 A to D, Diagrammatic ventral views of the abdominopelvic region of embryos and fetuses (sixth to ninth weeks) showing medial rotation and relocation of
the kidneys from the pelvis to the abdomen. A and B, Observe also the size regression of the mesonephroi. C and D, Note that as the kidneys relocate, they are supplied
by arteries at successively higher levels and that the hilum of the kidney is eventually directed anteromedially.
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Figure 12-20 Diagrams showing division of the cloaca into the urogenital sinus and rectum; absorption of the mesonephric ducts; development of the urinary bladder,
urethra, and urachus, and changes in the location of the ureters. A, Lateral view of the caudal half of a 5-week embryo. B, D, and F, Dorsal views. C, E, G, and H, Lateral
views. The stages shown in G and H are reached by the 12th week.
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Figure 12-22 Illustrations of urachal anomalies. A, Urachal cysts. The most common site is in the superior end of the urachus just inferior to the umbilicus. B, Two types of
urachal sinus are shown: One opens into the bladder and the other opens at the umbilicus. C, Patent urachus or urachal fistula connecting the bladder and the umbilicus.
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Figure 12-25 A, C, and E, Normal stages in the development of the infraumbilical abdominal wall and the penis during the fourth to eighth weeks. Note that mesoderm
and later muscle reinforce the ectoderm of the developing anterior abdominal wall. B, D, and F, Probable stages in the development of exstrophy of the bladder and
epispadias. B and D, Note that the mesenchyme fails to extend into the anterior abdominal wall anterior to the urinary bladder. Also note that the genital tubercle is
located in a more caudal position than usual and that the urethral groove has formed on the dorsal surface of the penis. F, The surface ectoderm and anterior wall of the
bladder have ruptured, resulting in exposure of the posterior wall of the bladder. Note that the musculature of the anterior abdominal wall is present on each side of the
defect. (Based on Patten BM, Barry A: The genesis of exstrophy of the bladder and epispadias. Am J Anat 90:35, 1952.)
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Figure 12-27 Schematic drawings illustrating development of the suprarenal glands. A, At 6 weeks, showing the mesodermal primordium of the fetal cortex. B, At 7
weeks, showing the addition of neural crest cells. C, At 8 weeks, showing the fetal cortex and the early permanent cortex beginning to encapsulate the medulla. D and E,
Later stages of encapsulation of the medulla by the cortex. F, Newborn infant showing the fetal cortex and two zones of the permanent cortex. G, At 1 year, the fetal
cortex has almost disappeared. H, At 4 years, showing the adult pattern of cortical zones. Note that the fetal cortex has disappeared and that the gland is much smaller
than it was at birth (F).
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Figure 12-29 A, Sketch of a 5-week embryo illustrating the migration of primordial germ cells from the umbilical vesicle (yolk sac) into the embryo. B, Three-dimensional
sketch of the caudal region of a 5-week embryo showing the location and extent of the gonadal ridges. C, Transverse section showing the primordium of the suprarenal
glands, the gonadal ridges, and the migration of primordial germ cells into the developing gonads. D, Transverse section of a 6-week embryo showing the gonadal cords.
E, Similar section at a later stage showing the indifferent gonads and paramesonephric ducts.
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Figure 12-31 Schematic illustrations showing differentiation of the indifferent gonads of a 5-week embryo (top) into ovaries or testes. The left side of the drawing shows
the development of testes resulting from the effects of the testis-determining factor (TDF) located on the Y chromosome. Note that the gonadal cords become
seminiferous cords, the primordia of the seminiferous tubules. The parts of the gonadal cords that enter the medulla of the testis form the rete testis. In the section of the
testis at the bottom left, observe that there are two kinds of cells, spermatogonia, derived from the primordial germ cells, and sustentacular or Sertoli cells, derived from
mesenchyme. The right side shows the development of ovaries in the absence of TDF. Cortical cords have extended from the surface epithelium of the gonad and
primordial germ cells have entered them. They are the primordia of the oogonia. Follicular cells are derived from the surface epithelium of the ovary.
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Figure 12-33 Schematic drawings illustrating development of the male and female reproductive systems from the genital ducts and urogenital sinus. Vestigial structures
are also shown. A, Reproductive system in a newborn male. B, Female reproductive system in a 12-week fetus. C, Reproductive system in a newborn female.
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Figure 12-34 A, Sketch of a ventral view of the posterior abdominal wall of a 7-week embryo showing the two pairs of genital ducts present during the indifferent stage of
sexual development. B, Lateral view of a 9-week fetus showing the sinus tubercle on the posterior wall of the urogenital sinus. It becomes the hymen in females and the
seminal colliculus in males. The colliculus is an elevated part of the urethral crest on the posterior wall of the prostatic urethra.
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Figure 12-36 Early development of the ovaries and uterus. A, Schematic drawing of a sagittal section of the caudal region of an 8-week female embryo. B, Transverse
section showing the paramesonephric ducts approaching each other. C, Similar section at a more caudal level illustrating fusion of the paramesonephric ducts. A
remnant of the septum that initially separates them is shown. D, Similar section showing the uterovaginal primordium, broad ligament, and pouches in the pelvic cavity.
Note that the mesonephric ducts have regressed.
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Figure 12-37 Development of the external genitalia. A and B, Diagrams illustrating the appearance of the genitalia during the indifferent stage (fourth to seventh weeks).
C, E, and G, Stages in the development of male external genitalia at 9, 11, and 12 weeks, respectively. To the left are schematic transverse sections of the developing
penis illustrating formation of the spongy urethra. D, F, and H, Stages in the development of female external genitalia at 9, 11, and 12 weeks, respectively.
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Figure 12-40 Schematic lateral views of the female urogenital system. A, Normal. B, Female pseudohermaphrodite caused by congenital adrenal hyperplasia (CAH).
Note the enlarged clitoris and persistent urogenital sinus that were induced by androgens produced by the hyperplastic suprarenal glands.
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Figure 12-44 Various types of uterine anomaly. A, Normal uterus and vagina. B, Double uterus (Latin, uterus didelphys) and double vagina (Latin, vagina duplex). C,
Double uterus with single vagina. D, Bicornuate uterus (two uterine horns). E, Bicornuate uterus with a rudimentary left horn. F, Septate uterus. G, Unicornuate uterus.
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Figure 12-46 A to F, Congenital anomalies of the hymen. The normal appearance of the hymen is illustrated in A and in the inset photograph. Inset, Normal crescentic
hymen in a prepubertal child. (Courtesy of Dr. Margaret Morris, Associate Professor of Obstetrics, Gynaecology and Reproductive Sciences, Women's Hospital and
University of Manitoba, Winnipeg, Manitoba, Canada.)
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Figure 12-47 Formation of the inguinal canals and descent of the testes. A, Sagittal section of a 7-week embryo showing the testis before its descent from the dorsal
abdominal wall. B and C, Similar sections at approximately 28 weeks showing the processus vaginalis and the testis beginning to pass through the inguinal canal. Note
that the processus vaginalis carries fascial layers of the abdominal wall before it. D, Frontal section of a fetus approximately 3 days later illustrating descent of the testis
posterior to the processus vaginalis. The processus vaginalis has been cut away on the left side to show the testis and ductus deferens. E, Sagittal section of a newborn
male infant showing the processus vaginalis communicating with the peritoneal cavity by a narrow stalk. F, Similar section of a 1-month-old male infant after obliteration
of the stalk of the processus vaginalis. Note that the extended fascial layers of the abdominal wall now form the coverings of the spermatic cord.
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Figure 12-48 Possible sites of cryptorchid and ectopic testes. A, Positions of cryptorchid testes, numbered in order of frequency. B, Usual locations of ectopic testes.
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