MULTICELLULAR ORGANIZATION: ONTOGENETIC 21 



chief fundaments of its future nervous, muscular, and other organization 

 at the upper pole. 



This might not appear to be so primitive a distinction as the early 

 evidence of bilateral symmetry seen or inferred in the oosperms of other 

 animals were it not for the fact that it may be compared functionally 

 with the early taxonomic associations of individual cells mentioned in 

 the preceding chapter and serving here as a suggestion of phylogenetic 

 history. In these, the use of the lower surface of the mass for the acqui- 

 sition of food may be compared to the storing of food in the lower part of 

 the body of the dividing ovum and to the subsequent development of 

 this surface by imagination into the chief digestive organs of the body. 

 This imagination of the lower body surface is known as gastrulation. 

 By this process the body mass of the young embryo becomes arranged in 

 an upper and a lower layer which are called respectively the ectoderm and 



FIG. 1 8. Transverse section of very young cat embryo: 

 showing ectoderm, mesoderm, and endoderm. X 400. 



endoderm. Where the animal is highly differentiated and a somewhat 

 more complete early differentiation is needed, a third layer is formed 

 between these two and is called the mesoderm (Fig. 18). 



In such an early differentiation we have certain groups of tissues 

 represented by the three layers, and to this extent the layers are appar- 

 ently homologous in many groups of nearly related animals. But ho- 

 mology breaks down to such an extent when details are examined that it 

 changes into an analogy when some larger groups are compared, 'and we 

 come to see that the analogy is based upon differentiations that are re- 

 sponses to particular conditions under which any embryos, or the embryos 

 of any group, must develop, rather than a fixed type of development which 

 is similar because of blood relationship. 



Protoplasm is slow to change its methods, however, and blood rela- 

 tionship, or common descent, probably does in a large degree determine 

 many similarities of embryological development. The moment, how- 

 ever, that we attempt to compare the wider groups, we must recognize 

 the possibility that even a large number of similar processes may proceed 



