3l8 COMPARATIVE ANATOMY 



vitelline duct, which disappears during the second month. Becoming at 

 first too long for the body cavity, a loop of the intestine pushes down into 

 the umbilical cord. In a six weeks' embryo, the beginning of a cecum. is 

 indicated by a swelling posterior to the vitelline duct. Later a horizontal 

 septum grows backward to divide the cloaca into a dorsal rectum and a 

 ventral urogenital sinus. Thus an important evolutionary change is 

 briefly repeated in ontogenesis. The septum forms the perineum of the 

 adult. During the second month, an anal canal is formed by the invagina- 

 tion of an ectodermal proctodeum and the rupture of the cloacal mem- 

 brane. The elongation and torsion of the intestine continue, and result in 

 the many convolutions of the small intestine and the ascending, trans- 

 verse, and descending portions of the colon. (Figs. 261, 266) 



The four layers of the intestinal wall develop as has been described for 

 the stomach. During the second month, the lumen of the small intestine 

 become temporarily occluded. Muscle fibers first appear in the second 

 month, and intestinal glands during the third month. 



History of the Intestine. The intestine as a region for the digestion 

 and absorption of food is present in the great majority of animals from 

 flatworms to man. An anal aperture makes its first appearance in flat- 

 worms, some of which, indeed, have two ani. Can the intestines and 

 anal apertures of invertebrates be homologized with structures having the 

 same name in vertebrates? Is an anus which Hes near the mouth, as in 

 cephalodiscus, rhabdopleura, and in mofluscoids, homologous with that 

 which, as in vertebrates, lies near the posterior end of the body? In view 

 of the present uncertainty in regard to the ancestry of vertebrates, it is 

 obviously hazardous to-day to attempt to homologize any vertebrate 

 structure with any organ of invertebrates. 



We may take as a test case the question of the origin of the vertebrate 

 anus. Is the vertebrate anus derived directly from that of any inverte- 

 brate type? Can we homologize the anus of cyclostomes and some 

 urodeles, which develops directly from the embryonic blastopore, with 

 that of an elasmobranch the anus of which does not develop from the 

 blastopore? Ignoring this difference in origin, Dohrn suggested that the 

 vertebrate anus, like the vertebrate mouth, may have been a new structure 

 formed by the coalescence of a pair of gill shts. The suggestion was made 

 notwithstanding the fact that in no chordate do gifl shts extend as far 

 back as the anus. The fact that, assuming amphioxus as a primitive 

 form, the lower the chordate, the larger the number of gill shts, seemed to 

 Dohrn a sufficient foundation for the hypothesis. That the vertebrate 

 anus is secondary has seemed further attested by the fact that there is a 

 postanal gut in vertebrate embryos and that the definitive anus arises 

 relatively late in ontogenesis. Vertebrate morphologists generally 

 regard the anus of vertebrates as homologous throughout the group not- 



