178 ABTHUR E. SHIPLEY. 



of the dorsal wall of the mesenteron and the epiblast, and 

 ventrally by the hypoblastic yolk-cells which are in contact 

 ■with the epiblast over two thirds of the embryo. Subse- 

 quently, but at a much later date, the mesoblast is completed 

 ventrally by the downgrowth on each side of these mesoblastic 

 plates. This takes place at a comparatively early stage in the 

 head and that part of the trunk lying in front of the liver. 

 In the posterior part, which remains swollen with yolk, the 

 ventral completion of the mesoblast is delayed. 



The first formation of the mesoblastic plates appears to 

 take place by a differentiation of the hypoblastic yolk-cells in 

 situ, and not from invaginated cells (figs. 12 and 13). The 

 subsequent downward growth is brought about by the cells 

 proliferating along the free ventral edge of the mesoblast, 

 these cells then growing ventralwards, pushing their way 

 between the yolk-cells and epiblast (fig. 11). 



This account of the origin of the mesoblast differs from that 

 given by Scott. He describes the mesoblast as arising from 

 two sources — (1) cells which are derived from the invagina- 

 tion of the blastoderm, (2) the outermost layer of the hypo- 

 blastic yolk-cells, which, according to Scott, split off from 

 the remainder, and form a ventral sheet which completes the 

 mesoblast in that side of the body. The mesoblast in the 

 head is derived only from the first source, as by the time it is 

 completed ventrally the head is raised above the yolk-con- 

 taining parts. 



Shortly before the development of the head fold raises the 

 head from the yolk-bearing part of the embryo, the neural 

 plate becomes evident in the exterior. It extends as a low 

 ridge from the anterior lip of the blastopore to just in front of 

 the blind anterior end of the mesenteron, over two thirds of 

 the circumference of the embryo. 



The blastopore is always visible at the posterior end of the 

 neural plate. Schultze has given a very complete set of figures 

 of the exterior of the embryo. As his figures show, with the 

 elongation of the embryo the anterior end curves round and 

 overlaps the posterior, thus obscuring the blastopore. Fig. 10 



