370 



THE GENITOURINARY SYSTEM 



bladder and the nerve-supply of the large intestine are 

 practically identical (p. 284). 



While the partition of the cloaca is being carried out, the 

 ventral wall loses its mesoderm in a part of its extent, and this 

 area, in which the endoderm of the gut comes into apposition 

 with the ectoderm of the body-wall, is termed the cloacal 

 membrane. The tailward-growing septum meets the cloacal 

 membrane and separates it into a ventral, genito-urinary part 

 and a dorsal, anal part, which closes in the gut segment. 



4 7 



I. II. Ill 



FIG. 129. The Development of the Bladder. 



In I., the cloacal membrane is just beginning to form. In II., it is very extensive, and 

 the cloaca is being divided into ventral and dorsal portions. In III., the sub-division 

 of the cloaca is complete and the uro-genital and anal membranes have ruptured. 



1. Hind-gut. 



2. Allantois. 



3. Cloacal membrane. 



4. Cloaca. 



5. Genital tubercle. 



6. Ventral, urinary, part of 



cloaca. 



7. Dorsal, gut, part of cloaca. 



8. Perineal orifice of uro- 



genital sinus. 



9. Anal orifice. 



Under normal conditions these membranes break down 

 during the third month of foetal life. The anal membrane 

 has already been considered (p. 287). The genito-urinary 

 membrane breaks down caudal to the genital tubercle, which 

 consists of a heaping up of the surface ectoderm at the 

 cephalic extremity of the membrane (Fig. 129). This 

 perineal orifice persists in the female, but it undergoes certain 

 changes in the male. 



Two elevations, termed the labio-scrotal folds, form one on 

 each side of the genital tubercle and grow tail wards. In the 



