Henry Gray (18251861). Anatomy of the Human Body. 1918.
junction of the hind-gut and allantois. This bend becomes dilated into a pouch, which constitutes the entodermal cloaca; into its dorsal part the hind-gut opens, and from its ventral part the allantois passes forward. At a later stage the Wolffian and Müllerian ducts open into its ventral portion. The cloaca is, for a time, shut off from the anterior by a membrane, the cloacal membrane, formed by the apposition of the ectoderm and entoderm, and reaching, at first, as far forward as the future umbilicus. Behind the umbilicus, however, the mesoderm subsequently extends to form the lower part of the abdominal wall and symphysis pubis. By the growth of the surrounding tissues the cloacal membrane comes to lie at the bottom of a depression, which is lined by ectoderm and named the ectodermal cloaca(Fig. 991).
FIG. 990 Diagrams to illustrate the development of the greater omentum and transverse mesocolon. (See enlarged image)
FIG. 991 Tail end of human embryo from fifteen to eighteen days old. (From model by Keibel.) (See enlarged image)
FIG. 992 Cloaca of human embryo from twenty-five to twenty-seven days old. (From model by Keibel.) (See enlarged image)
The entodermal cloaca is divided into a dorsal and a ventral part by means of a partition, the urorectal septum(Fig. 992), which grows downward from the ridge separating the allantoic from the cloacal opening of the intestine and ultimately fuses with the cloacal membrane and divides it into an anal and a urogenital part. The dorsal part of the cloaca forms the rectum, and the anterior part of the urogenital sinus and bladder. For a time a communication named the cloacal duct