152 



CHARLES R. STOCKARD 



Fig. 34 A camera lucida outline of the caudal region of the yolk-sac in a 

 96 hour embryo with the ectodermal cell borders made visible by mercury fix- 

 ation. An island of blood corpuscles is indicated by small circles which give some 

 idea of the minute size of the erythrocytes as compared with the huge ectodermal 

 cells. 



yolk to reach its ventral surface. At various places on the 

 posterior and ventral yolk surfaces the cells collect into groups 

 and become less active, although their movement does not 

 entirely cease. These groups constitute early blood islands. 

 They are at first not surrounded by endothelial cells but later 

 become enclosed or taken into the ends of the incipient vessels 

 as briefly described above. 



When the circulation first begins on the yolk-sac of a normal 

 embryo a great many of these islands are present, but are more 

 or less isolated or out of the channels through which th^ fluid is 



