DEVELOPMENT OF THE INTESTINAL CANAL. 



321 



human intestinal canal develops from the simple gastiaila 

 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 Amphioxns (in longitudinal secti(n): d, 

 I'rimitive intestine; 0, primitive month; i, intestinal layer; e, skin-layc^o 



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

 primitive intestine (d) and primitive mouth (0) are filled up by 1\\2 cells of 

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



« 



between the original p;:imary intestine ("the primitive 

 intestine, or 'protogastev ") of the Skull-less Animals 

 {Acrania), and the differentiated or secondary intestine 

 ( " after intestine, or metagaster " ) 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 



