338 



GENERAL EMBRYOLOGY 



and mesoderm are both derived from endoderm and the process 

 of involution is to be regarded as a true gastrulation process. 



But only the lesser part of the mesoderm is formed in the way 

 just described. This part of the mesoderm is known as the 

 gastral or parachordal, or axial mesoderm. If we trace the 

 mesoderm folds, just described, posteriorly, we can follow them 

 into the region of the germ ring or blastopore lip which has now 

 become considerably thickened on ac- 

 count of its contraction, and consists of 

 a more or less undifTerentiated mass of 

 cells. This mass now passes almost en- 

 tirely around the blastopore, laterally 

 and toward the ventral side. The 

 rapid proliferation of the cells of the 

 germ ring has early led to the disap- 

 pearance of the original simple epithe- 

 lial arrangement of its cells (Fig. 153), 

 but as it moves backward, with the 

 elongation of the gastrula, it leaves 

 behind it (i.e., anteriorly from it) its cell 

 products, which rapidly become differ- 



entiated ^ C6rtam larS - On the 



FK,153.-Frontal section 



through larva of Amphioxus, surface of the embryo a layer is left 



with six pairs of mesodermal -u r, i- ,1 , ,-, .-, 



somites, at the level of the wnlcn 1S directly continuous with the 

 notochord and somites, ectoderm of the original gastrula de- 



After Cerfontaine. a, , -, 



archenteron; e, enterocoei; rived irom the animal hemisphere of 



ir m tode' ^'af t: s nd th *. blastula. Another layer is differ- 



entiated lining the archenteron and 



continuous with the layer already there, and consisting of a ven- 

 tral region continuous with the invaginated layer or true gut 

 endoderm, a dorsal region continuous with the involuted layer 

 forming the rudiment of the notochord, while dorso-laterally, 

 between these two regions, the inner sheet is continuous with 

 the mesodermal rudiments described above. In other words, 

 put of the germ ring there are gradually differentiated, true cover- 

 ing ectoderm, gut endoderm, chordal cells, and mesoderm. The 

 mesoderm formed in this way, directly from the germ ring, is 



