ALIMENTARY CANAL. 



767 



tually appear a dilatation of the mesenteron arises, which comes 

 in contact with the external skin (fig. 28 E, an}. This dilatation 

 becomes the hypoblastic section of the cloaca. It communicates 

 behind with the postanal gut (fig. 424 D), and in front with the 

 intestine ; and may be defined as the dilated portion of the alimen- 

 tary tract which receives the genital and urinary ducts and opens 

 externally by the proctodczum. 



In Acipenser and Amphibia the cloacal region is indicated 

 as a ventral diverticulum of the mesenteron even before the 

 closure of the blastopore. It is shewn in the Amphibia at an 

 early stage in fig. 73, and at a later period, when in contact with 

 the skin at the point where the anal invagination is about to 

 appear, in fig. 420. 



FIG. 420. LONGITUDINAL SECTION THROUGH AN ADVANCED EMBRYO OF 



BOMBINATOR. (After Gotte.) 



m. mouth ; an. anus ; /. liver ; ne. neurenteric canal ; me. medullary canal ; ch. 

 notochord ; pn. pineal gland. 



In the Sauropsida and Mammalia the cloaca appears as a 

 dilatation of the mesenteron, which receives the opening of the 

 allantois almost as soon as the posterior part of the mesenteron 

 is established. 



The eventual changes which it undergoes have been already 

 dealt with in connection with the urinogenital organs. 



Intestine. The region in front of the cloaca forms the 

 intestine. In certain Vertebrata it nearly retains its primitive 

 character as a straight tube ; and in these types its anterior 

 part is characterised by the presence of a peculiar fold, which in 

 a highly specialised condition is known as the spiral valve. 

 This structure appears in its simplest form in Ammocoetes. It 



