70 ANIMAL PHYSIOLOGY. 
The fowl’s ovum then belongs to the class, a portion of which 
alone segments and develops into the embryo (meroblastic), in 
contradistinction to what happens in the mammalian ovum, the 
whole of which undergoes division (holodlastic); a distinction 
which is, however, superficial rather than fundamental, for in 
reality in the fowl’s egg the whole of the original ovum does 
Fie. 74.—Sections of ovum of a rabbit, illustrating formation of the plastodermic vesicle (after 
E. Van Beneden). A, B,C, D,are ova in successive stages of development. zp, zona pellu- 
cida ; ect, ectomeres, or outer cells ; ent, entomeres, or inner cells. 
segment. This holoblastic character of the mammalian ovum 
and its resemblance to the segmentation of those invertebrate 
forms previously described may become apparent from an ex- 
amination of the accompanying figures. 
Weshall return to the development of the mammalian ovum 
later ; in the mean time we present the main features of devel- 
opment 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 
