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MESONEPHROS

The number of segments occupied by the mesonephros is reduced, extending approximately from T-9 to L-3. The corpuscles and tubules in the cranial segment begin to degenerate.

The mesonephric duct is narrow at its cranial end but enlarges as it courses caudally on the ventral aspect of the mesonephros to terminate in the urogenital sinus.

The female (paramesonephric or Müllerian) duct, which will become the main genital duct in the female, begins as an invagination of the coelomic epithelium on the ventrolateral surface of the urogenital ridge. Initially the invagination is a solid bud which later canalizes. The duct grows caudomedially toward the pelvic part of the urogenital sinus. As it approaches the sinus, it meets and fuses with its counterpart on the other side.


METANEPHROS (KIDNEY)

The primitive renal pelvis bifurcates into cranial and caudal major calyces within the metanephrogenic mass. Each major calyx branches deeper into the mass thereby forming collecting tubules. The distal end of each tubule is covered by a condensation called the renal capsule that is derived from the cells of the metanephrogenic mass.

The metanephros attains its definitive position ascending to the level of the T-12 to L-2 segments.

The proximal segment of the metanephric diverticulum becomes the ureter. As the horn of the urogenital sinus is absorbed into the main chamber, the entrance of the ureter is relocated lateral to the entrance of the mesonephric duct.

UROGENITAL SINUS

The urorectal septum completely separates the primitive urogenital sinus from the more dorsal rectum. The resulting definitive urogenital sinus is divided into three portions: a cranial, dilated bladder portion, an intermediate, narrow pelvic portion and a caudal phallic portion. The phallic portion is essentially a vertical slit between the urogenital folds and is separated from the exterior by the urogenital part of the cloacal membrane.

The allantois is continuous with the apex of the bladder portion of the urogenital sinus. As the lumen in the junction area disappears, the allantois is replaced by a solid, ligamentous structure called the urachus or median umbilical ligament.

GONAD (INDIFFERENT STAGE)

The gonad in the indifferent stage is a lobular mass extending from approximately T-10 to L-2 segments. It attaches to the medial side of the mesonephros by the gonadal mesentery where it comes into contact with some of the mesonephric tubules.

The primordial germ cell from the yolk sac reside in the gonadal ridge. As they enter the gonad, the surface cells of the ridge proliferate into gonadal cords that extend into the underlying mesenchyme around the germ cells.

EXTERNAL GENITALIA

Since the external genitalia is identical in the male and female, it is not yet possible to distinguish the sex from the surface features.

The cloacal membrane remains in the midline and is subdivided into urogenital and anal parts. The urogenital part separates the phallic part of the urogenital sinus from the urogenital groove. The anal part separates the rectum from the proctodeum.

The urogenital groove is flanked on each side by a prominent urogenital fold. The proctodeum is flanked on each side by an anal fold.

The genital tubercle in the midline and the more lateral labioscrotal swelling become prominent raised areas.

Source: Atlas of Human Embryos.