DEVELOPMENT OF THE INTESTINAL CANAL. 



321 



human intestinal canal develops from the simple gastrula 

 and which is similar to that in other Mammals, can there- 

 fore be only correctly understood when it is considered in 

 the light of Phylogeny. We must, accordingly, distinguish 



Fig. 276. — Archigastrula of Amphioxus (in longitudinal section) : d, 

 primitive intestine ; 0, primitive mouth ; i, intestinal layer ; e, skin-layer. 



Fig. 277. — Amphigastrula of Mammal (in longitudinal section). The 

 primitive intestine (d) and primitive mouth (o) are filled up by the cells of 

 the intestinal layer (i) ; e, skin-layer. 



between the original primary intestine ("the primitive 

 intestine, or protogaster ") of the Skull-less Animals 

 (Acrania), and the differentiated or secondary intestine 

 ( " after intestine, or mctagaster " ) of the Skulled Animals 

 (Craniota). The intestine of the Amphioxus (the repre- 

 sentative of the Acrania) forms no yelk-sac, and develops, 

 palingenetically, from the entire primitive intestine of the 

 gastrula. The intestine of the" Skulled Animals, on the 

 other hand, has a modified, kenogenetic form of evolution, 

 and differentiates at a very early period into two different 

 parts : into the permanent secondary intestine, which alone 



