The Development of the Frog 29 



dorsal lip of the blastopore has first appeared, and even 

 just prior to that time. The innermost of those cells 

 forming the ring around the egg are the cells that become 

 the mesoderm. These cells are carried up to the median 

 dorsal line of the embryo by the closure of the blasto- 

 pore. They will then be found forming a layer or sheet 

 of cells that separates itself on the outer side from the 

 thick layer of small ectodermal cells (that has been simul- 

 taneously lifted up) and that is separated on the inner 

 surface, but not very sharply, if at all, from the dorsal 

 and dorso-lateral walls of the archenteron." 



This layer of mesoblast, lying between the 

 ectoblast above and the entoblast below, is de- 

 scribed by Morgan as being continuous across 

 the dorsal side of the embryo, but it is more 

 often said to consist of two lateral portions, sep- 

 arated along the mid-dorsal line by the noto- 

 chord, which is formed at about the same time. 



The two plates of mesoblast rapidly extend 

 towards the mid-ventral line, where they fuse 

 and thus form a continuous layer under the 

 ectoblast (Figs. 10 and 13). 



Soon after its formation as a distinct layer, 

 the mesoblast separates, beginning on each 

 side of the notochord, into two layers, one 

 lying next to the ectoblast and known as the 

 somatopleure, and one lying next the ento- 

 blast, or yolk, and known as the splanchno- 



