hevelopjiext of the intestixal panal. 



321 



liuman 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, accoixlingly, distinguish 



Fig. 276. — Archigastrnla of Amphioxns (in longitudinal fcctiou) : c7, 

 primitive intestine ; 0, primitiTe mouth ; 1, 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 (/) ; e, skin-layer. 



between the original primary intestine (" the primitive 

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

 (Acranm), and the differentiated or secondary intestine 

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

 (Cramoia). 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 secondarj^ intestine, which alone 



