72 



COMPARATIVE PHYSIOLOGY. 



and its resemblance to the segmentation of those invertebrate 

 forms previously described may become apparent from an ex- 

 amination of the accompanying figures. 



!Pig. 70. — Sections of ovum of a rabbit, illustrating formation of the blastodermic vesi- 

 cle (after E. Van Beneden). A, B, G, D, are ova in successive stages of develop- 

 ment, z^, zona pellncida; ecit ectomeres, or outer cells; ent, entomeres, or inner 

 cells. 



We shall return to the development of the mammalian ovum 

 later ; in the mean time we present the main features of develop- 

 ment in the bird. 



Remembering that the development of the embryo proper 

 takes place within the pellucid area only, we point out that the 

 area opaca gradually extends over the entire ovum, inclosing the 

 yelk, so that the original disk which lay like a watch-glass on 

 the rest of the ovum, has grown into a sphere. That portion 

 of this area nearest the pellucid zone (area vasculosa) develops 

 blood-vessels that derive the food-supplies, which replenish the 

 blood as it is exhausted, from the hypoblast of the area opaca. 



