THE DIGESTIVE APPARATUS 



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 Miillerian ducts open into its ventral portion. The cloaca is, for a time, shut 



I 'ill /ill I'll II III 



Liver 



I.n-ci- 



Lfxner ninrnfiim . 

 It ill-Kit mill 



i.i-nil, i- iuii, nlinn 

 Tr<iiiKi-i' i-.tr m 



1 'i-iinxnene colon - 



I.: V/- Illllriltltlll 



-- Bursa omentalis 

 ' ' Htiimacfi 



Pancreas 



Obliterated part of mesogastrium 



Duodenum 



Transverse culun 



Small intestine 



Small ii\ 



Mesentery 

 Fio. 990. Diagrams to illustrate the development of the greater ornontum and transverse mesocolon. 



oil' from the anterior by a membrane, the cloacal membrane, formed by the apposi- 

 tion 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. 1)91). 



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 



Wolffian dw,t 



Noloclwrd 



FIG. 991. Tail end of human embryo from fifteen to 

 eighteen days old. (From model by Keibol.) 



KKI. 



of human embryo from 

 seven days old. (From 



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 uro- 

 ?enital sinus and bladder. For a time a communication named the cloacal duct 



