214 BIOLOGY: GENERAL AND MEDICAL 



closed between the two chief blastodermic layers they 

 also become separated from the parent blastodermic 

 layers and differentiated as a third blastodermic layer. 



A transverse section of the germinal disc at the time 

 of the formation of the mesoderm, which coincides fairly 

 well with the period of the formation of the primitive 

 streak and groove, is shown in the illustration. 



The future development of such meroblastic eggs 

 depends upon their character. If the eggs are enclosed 

 in shells, development is complicated by the formation 



FIG. 90. FIG. 91. 



FIG. 90. Ovum of the bat, showing vacuolation of the segmented egg to 



form the blastodermic cavity. X 500. (Van Beneden.) 



FIG. 91. Ovum of the bat, showing vacuolation of the segmented egg to form the 

 blastodermic cavity. X 600. (Van Beneden.} 



of a membrane investing the embryo the amnion; if 

 the eggs are without shells, as those of fishes, there are 

 no membranes, and development is less complicated. 

 In both cases the growth of the embryo continues by 

 folding, convergence, and concrescence of the cellular 

 germinal plate, more and more completely separating 

 the embryo from its yolk. 



Now, leaving the meroblastic eggs of the fishes, rep- 

 tiles, birds, and the lowest mammals, we return once 

 more to holoblastic eggs as we find them in mammals. 



