A L1MEXTAR Y CA JVA I. 



631 



Fiu. 420. LONGITUDINAL SECTION THROUGH AN AD- 

 VANCED EMBRYO OF BoMBiNATOR. (After GStte.) 



in. mouth; an. anus; Z. liver; ne. neurenterio 

 canal; me. medullary canal ; ch. notochord; pn. pineal 

 gland. 



In the Sauropsida and Mammalia the cloaca appears as a dila- 

 tation of the mesen- 

 teron, which receives 

 the opening of the 

 allantois almost as 

 soon as the posterior 

 part of the mesente- 

 ron is established. 



The eventual 

 changes which it un- 

 dergoes have been 

 already dealt with in 

 connection with the 

 urinogenital organs. 



Intestine. The 

 region in front of the 

 cloaca forms the in- 

 testine. 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 Ammoccetes. It there 

 consists of a fold in the wall of the intestine, giving to the lumen 

 of this canal a semilunar form in section, and taking a half 

 spiral. 



In Elasmobranchii a similar fold to that in Ammoccetes first 

 makes its appearance in the embryo. This fold is from the first 

 not quite straight, but winds in a long spiral round the intestine. 

 In the course of development it becomes converted into a strong- 

 ridge projecting into the lumen of the intestine (fig. 388, I). The 

 spiral it makes becomes much closer, and it thus acquires the form 

 of the adult spiral valve. A spiral valve is also found in Chima?ra and 

 Ganoids. No rudiment of such an organ is found in the Teleostei, 

 the Amphibia, or the higher Vertebrata. 



The presence of this peculiar organ appears to be a very primitive 

 Vertebrate character. The intestine of Ascidians exhibits exactly the 

 same peculiarity as that of Ammoccetes, and we may probably con- 

 clude from embryology that the ancestral Chordata were provided 

 with a straight intestine having a fold projecting into its lumen, to 

 increase the area of the intestinal epithelium. 



In all forms in which there is not a spiral valve, with the excep- 

 tion of a few Teleostei, the intestine becomes considerably longer 

 than the cavity which contains it, and therefore necessarily more or 

 less convoluted. 



The posterior part usually becomes considerably enlarged to form 

 the rectum or in Mammalia the large intestine. 



In Elasmobranchii there is a peculiar gland opening into the 



