342 



TEXT-BOOK OF EMBRYOLOGY. 



in development, remains more slender and forms the vermiform appendix 

 (Fig. 305). 



As has already been mentioned, the primitive gut ends blindly in the caudal 

 end of the embryo (Fig. 284). The anal opening is a secondary formation. 

 On the ventral side of the caudal end of the body there is formed a depression 

 known as the anal pit. The mesoderm at the bottom of the pit becomes thin- 

 ner until the ectoderm comes in contact with the entoderm on the ventral side 

 of the gut, thus forming the anal membrane. The area of contact is not at the 



FIG. 305. Drawing from a model of the small intestine in the adult. Ventral view. Mall. 



The intestinal coils are shown in the usual relative position. The numbers indicate the coils derived 



from the primary bends in the fcetus as shown in Figs. 302 and 303. 



extreme end of the gut, but a short distance toward the allantoic duct. In the 

 meantime, the urogenital ducts come to open into that portion of the gut which 

 lies just cranial to the anal membrane. The gut enlarges in this region to 

 form the cloaca. The latter becomes separated by the urorectal fold into a 

 dorsal portion, the rectum, and a ventral portion, the urogenital sinus (Figs. 361 

 and 363). At about the time of separation (embryos of about 14 mm. or 

 thirty-six to thirty-eight days) the anal membrane ruptures and the anal open- 



