THE UROGENITAL SYSTEM. 



121 



on the ventral aspect of the hind gut (Fig. 108, p. 136). There 

 is a projection of the gut behind the proctodaeal depression. 

 This is the post-anal gut. It commonly disappears, no trace of 



cloacal sept, imperfect 



™" rectum 



vagina 



"ifg^^—anal plate 



anal dep. 



perineal sept, not united 

 to cloacal septa. 



Fig. 97. —A case in which the Rectal part of the Anal Plate has persisted and the 

 Cloacal Septum has failed to fuse with the Perineal Septum. 



it being found after the 1st month, but remnants of it are said 

 to give rise to ano-coccygeal tumours and cysts. 



The Neurenteric Canal — Ano-coccygeal tumours are also 

 believed to arise from remnants of the neurenteric canal. The 

 neurenteric canal, or blastopore, it will be remembered (p. 8 9, 

 Figs. 68, 75 and 158) is a communication of the cavity of the 

 hypoblast with the surface of the epiblast. It probably represents 

 the point at which an area of cells was invaginated within the 

 epiblast to form the hypoblast. As it opens at the anterior end 

 of the primitive streak, which afterwards is included in the 

 posterior end of the neural groove, such a canal, if it persisted, 

 might be expected to perforate the coccyx and communicate 

 between the canal of the notochord and filum terminale. No 

 remnant of this canal has been found after the 1st month of 

 foetal life. 



The Uro-genital Cleft or Depression is the anterior part of the 

 proctodaeal or primitive perineal depression, cut off from the 

 posterior or anal part by the development of the perineal septa. 

 With the absorption of the cloacal membrane, the uro-genital 

 sinus opens in the cleft (Fig. 95.5). The cleft remains alike in 

 both sexes until the third month. It forms the greater part of 



