EMBRYOLOGY OF TADPOLE AND CHICK 



563 



These extend from the blastopore to almost directly opposite on the 

 dorso-lateral portion of the embryo. Here they turn sharply to pass 

 toward the midline where they meet to form the transverse neural fold. 

 It is this latter fold which marks the anterior limit of the medullary 

 plate. The median groove becomes more pronounced and is then called 

 the neural groove. 



The important points to bear in mind here are : 



Firstj that as in the chick and all chordate animals, so, too, in the 

 frog, the closing of the blastopore by confluence (extending in an antero- 

 posterior direction, on the dorsal aspect of the embryo) forms a definite 



Fig. 327. 



Frog embryos. A, from behind and above ; B, from in front ; C, slightly earlier 

 than A and B. an., proctodeum (the invagination from which anus will form) ; 

 blp., blastopore ; ga, gill arches ; gp., gill plate ; n/., neural fold ; a, sucker ; ap., 

 sense plate. (After Borradaile.) 



axis, which means that by this confluence, the germ-ring becomes the 

 axis. It is on each side of this axis that organs and important struc- 

 tures develop. 



Second, that gastrulation involves only the forming of two layers 

 (ectoderm and entoderm) from the single layered blastula. 



Third, that notogenesis includes all the processes involved in the 

 formation of the medullary plate, notochord, and mesoderm. 



Gastrulation is accomplished in the frog chiefly by a delamination 

 and rearrangement of the yolk cells and only to a slight extent by in- 

 vagination. The process of invagination is chiefly concerned in forming 

 the beginning of the notochord and the mesoderm in the dorsal and 

 dorso-lateral regions. These latter structures are not formed entirely, 

 however, by invagination, but also by material from the germ-ring which 



