144 EMBRYOLOGY. 



head- and tail-ends, the future dorsal and ventral sides of the body. 

 The blastopore (crescentic groove, primitive groove) marks the 

 posterior end. The ventral side is characterised by being the place 

 where the segmented or unsegmented yolk-material comes to He. 



C. The embryo with four germ-layers and a body-cavity. 



1. In all Vertebrates there are formed from the roof of the 

 ccelenteron two lateral evaginations of the inner germ-layer, by 

 means of which the ccelenteron is divided into a median cavity, the 

 secondary intestine, and two lateral cavities, the two body-sacs. 



2. The primary inner germ-layer is resolved in consequence of 

 this process of evagination into three parts : 



First, the epithelial lining of the intestinal tube (secondary 



inner germ-layer Darmdriisenblatt). 

 Secondly, the epithelial lining of the body-cavity, or the middle 



germ-layer, in which a parietal and a visceral layer are 



distinguishable. 

 Thirdly, the chorda, which takes its origin from the portion of 



the primary inner germ-layer which lies between the 



lateral evaginations from the roof of the ccelenteron. 



3. Two modifications of the process of evagination can be recog- 

 nised in the case of Vertebrates. 



(a) In Amphioxus the evaginations are small, numerous, and 

 segmentally arranged; provided from the first with a 

 cavity ; and, beginning in the fundus of the ccelenteron, 

 developed toward the blastopore. 



(&) In the remaining Vertebrates, instead of hollow sacs, there 

 grow out from the inner germ-layer two solid masses of 

 cells : 



(1) In the vicinity of the blastopore (primitive groove, 



peristomal mesoblast). 



(2) From here forward along the roof of the ccelenteron, 



at a slight distance from the median plane, at both 

 sides of the fundament of the chorda (gastral 

 mesoblast). 



The paired fundaments spread themselves out from 

 their place of origin between the two primary germ- 

 layers farther forward and ventralward. 



4. The three organs derived from the primary inner germ-layer 

 (middle germ-layer, fundament of the chorda, secondary inner germ- 

 layer) are separated from one another by constrictions. 



