48 GERMINAL LAYERS OF BLASTODERMA. 



mammal is that the former, not having to be hatched 

 in a womb, contains all the material necessary for the 

 development of the chick, whereas the latter contains 

 sufficient materials only for the formation of the first 

 traces of the embyro ; the materials for the further 

 development of the embryo and fcetus being" derived, 

 within the mother's womb, from her blood through 

 the medium of the placenta. 



To return to the ovum in the stage of globu- 

 lar blastoderma. In some animals, chiefly in- 

 vertebrate, though also in the lancelet, and probably 

 in some higher vertebrata as well, the surface is 

 beset with vibratile cilia. This planula-larva form 

 Haeckel considers to be, ontogenetically, a recapitu- 

 lation of a hypothetical many-cell protozoon which 

 he names PLAN.EA, and which he supposes first made 

 its appearance in the primordial time. In the next 

 stage of development the blastoderma becomes 

 separated into two distinct strata of cells, named the 

 primary germinal layers. The cells composing these 

 layers are different from each other in general aspect. 

 That they are different also in endowments and poten- 

 tialities as regards future development, we shall see. 

 r With the separation of the blastoderma into the two 

 ) J germinal layers, respectively named : the outer, 

 EXODERM (otherwise animal or serous layer), and 

 the inner, ENTODERM (otherwise vegetative or mucous 

 layer), an important step towards the fundamental 

 construction of the embyro is made. 



The ovum of man and the mammifera in this 



