THE LARGE INTESTINE 287 



subdivision is in progress, the mesoderm separating the ventral 

 wall of the cloaca from the ectoderm disappears over an area 

 which is termed the cloacal membrane. 



When the subdivision of the cloaca is completed, it is found 

 that the cloacal membrane also has been divided into two 

 corresponding parts, which are termed the urogenital and 

 anal membranes, respectively. The anal membrane becomes 

 depressed to form the proctodseum and it finally breaks 

 down, so that the anal canal opens on the surface of the body 

 (Fig. 103). During the subdivision of the cloacal membrane, 

 the mesoderm grows into the central area so as to separate the 

 urogenital and anal membranes from one another. If it grows 

 in between the ectodermal and endodermal layers of the anal 

 membrane, the membrane fails to break down and the condition 

 of atresia ani results. 



It will be seen from Fig. 103, III., that the lining of the 

 upper part of the anal canal is derived from entoderm, while 

 that of the lower part is derived from ectoderm. 



