(now a ventral outpouching of the hind gut) and are usually seen in the 17–20 day embryo, although possible primordial germ cells have been identified at 13 days [8]. The primordial germ cells are ectodermal in origin, having migrated to the allantoic diverticulum from the epiblast, and they retain two functional X chromosomes in contrast to the somatic cells, which possess only one functional X chromosome [9]. From here, they migrate through the mesoderm surrounding the hindgut and into the dorsal mesentery. The final destination is the gonadal ridge, which they reach at about 35 days (Figure 1.4).
Carnegie stage 9
The neural plate and the longitudinal neural ridges develop. The embryo flexes to accommodate the neural tube (Figures 1.5a–c) and in so doing reorients the primitive embryonic tissues and their relationship to each other. The endoderm of the dorsal part of the yolk sac is drawn into the ventral concavity of the embryo and is subdivided into foregut, midgut, and hindgut (Figure 1.6a,b). The hindgut appears about day 20 and is enclosed within the tail fold of the embryo. In this situation, the hindgut lies caudal to the rostral limit of the allantoic diverticulum and dorsal to the cloacal membrane. The mesoderm in the mid‐embryo region is divided into paraxial (surrounding the neural tube), lateral, and intermediate mesoderm. The intermediate mesoderm lies ventrally and lateral to the paraxial mesoderm, and differentiates medially into the gonadal ridge and laterally into the mesonephric region. The intermediate mesoderm at the rostral limit of the allantoic diverticulum extends dorsally and then caudally, in line with the curvature of the tail fold, dividing the hindgut into ventral and dorsal parts. As this division proceeds, the two parts of the hindgut remain in continuity with each other caudal to the advancing mesoderm of the urorectal septum. The caudal end of the hindgut is lined with endoderm and is known as the cloaca. On the ventral aspect of the cloaca there is a membrane which separates the endoderm from the surface ectoderm, the cloacal membrane. As development continues, a mesenchymal septum, the urogenital septum, migrates caudally (Figures 1.7a–c).
Figure 1.3 (a) The floor of the amniotic cavity, the dorsal surface of the bilaminar embryonic disc, revealing the primitive streak and notochord. (b) Intraembryonic mesoderm, generated by the primitive streak and interposed between the floor of the amniotic cavity and roof of the yolk sac, converts the bilaminar embryonic disc into a trilaminar disc. The buccopharyngeal and cloacal membranes remain bilaminar.
Figure 1.4 Migration of primordial germ cells into the genital ridge from the body stalk.
Figure 1.5 (a) As the neural tube is enclosed within the intraembryonic mesoderm, it lengthens and expands rostrally, causing a dorsal convexity and ventral concavity. (b) Further growth of the neural tube increases this curvature in the longitudinal plane (c) with the eventual formation of the head and tail folds.
Figure 1.6 (a) Midline section of the embryo after formation of the head and tail folds. (b) A transverse section of the mid‐embryo region after formation of the lateral folds.
Carnegie stage 11
When embryo flexion has been completed (day 24), the anterior limit of the cloacal membrane abuts on the base of the umbilical cord. On either side of the cloaca are the paired primordia of the genital tubercle (Figure 1.8a) which fuse as the cloaca retracts from the umbilical cord to form an anterior wall. Posterior to the tubercle and running laterally by the sides of the cloacal membrane are the cloacal folds, and lateral to these the genital swellings (Figure 1.8b).
Carnegie stages 13 and 14
The urogenital septum reaches the cloacal membrane at 30–32 days and fuses with the cloacal membrane, dividing the embryonic hindgut into the ventral (anterior) urogenital sinus and dorsal (posterior) rectum.
The cloacal membrane is also divided, the ventral part forming the urogenital membrane and the dorsal part forming the anal membrane. It eventually ruptures to form the urogenital and anal orifices. The genital folds develop from the anterior part of the cloacal folds, and the anal folds form the posterior component. (Figure 1.8c). The two primitive gonadal streaks proliferate to form a genital tubercle at the ventral tip of the cloacal membrane. These two genital tubercles grow further and must reach a critical mass, or only rudimentary structures will be formed. They then fuse to form the glans of the clitoris. Labioscrotal swellings and urogenital folds develop on each side of the cloacal membrane. As the urogenital septum reaches the cloacal membrane, the labioscrotal folds fuse posteriorly forming the perineal body, which separates the urogenital membrane from the anal membrane, but sexual differentiation at this stage is still indeterminate. Cells from cephalic mesonephric vesicles invade the coelomic epithelium on the medial aspect of the adjacent intermediate mesoderm to induce the formation of the indifferent gonad at 30–32 days [10].
Carnegie stages 15 and 16
The indifferent gonad begins to develop (day 35) on the medial aspect of the mesonephros by the invasion of three other cell types: the primordial germ cells, cells from the overlying coelomic epithelium, and the cells from the adjacent mesonephros. All cell types are probably essential to the proper differentiation of the gonad. The paramesonephric ducts (Müllerian ducts) appear at about 40 days. The precursor of each duct extends caudally as a solid rod of cells in the intermediate mesoderm, in close association with, and initially lateral to, the mesonephric (Wolffian) duct. The mesonephric duct has been shown experimentally both to induce the paramesonephric duct [11] and to guide its descent [12]. The growing caudal tip of the paramesonephric duct lies within the basement membrane of the mesonephric duct. As they descend, the paramesonephric ducts pass ventral to the mesonephric ducts and, coming into close association with one another, reach the posterior aspect of the urogenital sinus within the urorectal septum (Figure 1.9). The two paramesonephric ducts begin to fuse even before their growing ends reach the urogenital sinus [13]. As the urorectal septum reaches the cloacal membrane (day 30–32), the caudal end of the mesonephric duct, having already opened into the urogenital sinus, begins to form the ureteric bud and be incorporated into the posterior wall of the urogenital sinus. The portion of each duct incorporated