DEVELOPMENT OF PHEYGANIDS. 685 



both in number and size. Their nuclei have become enormous, 

 and it is only occasionally that one can distinguish protoplasm 

 surrounding them. In size and general appearance they closely 

 resemble granular yolk masses, and if their transition had not 

 been traced step by step one would not believe that they were 

 cells. Before the splanchnic mesoderm has completely sepa- 

 rated the yolk from the body cavity on the ventral side some 

 of the yolk-cells migrate into the body cavity, where they 

 arrange themselves irregularly along the sides of the body wall. 

 They are at once distinguished from the surrounding mesoderm 

 cells by their great size and amoeboid protoplasmic arms. At 

 first the cells are arranged singly and indefinitely along the 

 outer wall of the body; but later, by increase in numbers, 

 they become arranged in distinct groups, which will be described 

 more definitely in the next stage. 



Exceedingly large vesicular cells, and probably of the same 

 origin as those just described, are seen during this stage on the 

 ventral side of the oesophagus, where it comes in contact with the 

 yolk (PI. XXXVIc, fig. 22, en. c, and PI. XXXVIc, fig. 36, a). 



When the embryo is ready to hatch, these cells have disap- 

 peared, although their exact fate I was not able to determine 

 with certainty. 



In the latter part of this period the yolk-cells begin to 

 arrange themselves upon the walls of the yolk-sac. During 

 this process they become less distinct and gradually reduced 

 in numbers, owing to the migration of some of them into the 

 body cavity. In the anterior part of the yolk-sac it was ex- 

 tremely difficult to observe yolk -cells in the act of forming the 

 epithelial lining of the stomach, since the cells were both so 

 indistinct and few in number. On the other hand, it was com- 

 paratively easy to observe this in the posterior part of the yolk- 

 sac, since the cells were both distinct and numerous; the 

 epithelial lining of the stomach is probably formed first in the 

 neighbourhood of the proctodaeum, and then extends forwards 

 to the stomodseum. It is certain, however, that the yolk-cells 

 do not form a continuous sac until some time after the 

 formation of the mesodermic musculation of the yolk-sac. This 



