56 



TEXT-BOOK OF EMBRYOLOGY. 



tion of the significance of this first cell differentiation is evidenced by the fact 

 that he designated the two primary germ layers the "primitive organs" of the 

 body. 



It should be noted that with the completion of gastrulation certain important 

 landmarks in adult topography have been established. Thus the animal 



Segmentation cavity 



Micromeres 



Segmentation cavity 



Macromeres 



Invagination 



D 



Archenteron Anterior lip of blastopore 



Blastopore 



Post, lip of 

 blastopore 



Ectoderm Entoderm Ectoderm Entoderm 



FIG. 34. Gastrulation in Amphioxus. Hatschek, Bonnet. 



(micromere) pole is always the dorsum; the vegetative (macromere) pole 

 always the ventrum; the blastopore, being always caudal, differentiates the 

 tail end from the head end of the embryo. 



Gastrulation in Amphibians. 



This is modified as compared with gastrulation in Amphioxus by the 

 presence of a greater amount of yolk. A clear understanding of the modifica- 

 tions which this increased yolk content causes in the gastrulation of Amphibians, 



