120 HUMAN EMBRYOLOGY AND MORPHOLOGY. 



The cloacal membrane or plate (Fig. 95 A), formed of the applied 

 layers of epiblast and hypoblast between the perineal depression 

 and the cloaca, breaks down before the end of the 1st month of 

 foetal life. The uro-genital sinus then comes to open into the 

 uro-genital part of the depression, while at a rather later date the 

 rectum opens into the anal part. The lateral perineal ingrowths, 

 which divide the proctodaeum into uro-genital and anal parts, 

 complete the perineal body (see Fig. 98). 



Imperforate Anus. — This condition occurs not unfrequently, 

 and in many forms. It is due to a mal- development of the cloacal 



cloacal. sept. 



rectum 



anal plate 



depress, 

 uagma j 



perineal septum 



Fib. 96.— A case of Imperforate Anus due to a persistence of the Anal Plate. 



and perineal septa. In some cases it is merely the anal part of the 

 cloacal plate that fails to break down (Fig. 96). The white line, 

 situated at the lower ends of the columns of Morgagni, marks the 

 situation of the anal plate and the junction of the epiblast of the 

 anal depression with the hypoblast of the rectum. 



In over 50°/; of cases of imperforate anus the rectum ends 

 blindly two inches or more above the anus. In such cases the 

 lateral septa which divide the cloaca appear to have been mal- 

 placed and included the whole of the cloacal part of the hind gut 

 in the uro-genital canal. On the other hand the cloacal septa 

 may be incomplete, and the uro-genital sinus only partially 

 separated from the rectal. In such cases there is a communica- 

 tion between the rectum and termination of the vagina in the 

 female (Fig. 97), and the part of the urethra formed from the 

 uro-genital sinus in the male (Fig. 93). 



Post-anal Gut. — It will be observed that the anus is formed 



