BLASTJJLA, GASTRULA, AND GERM LAYERS 359 



development prior to the formation of the germ layers. These 

 structures are by no means the earliest constant embryonic 

 differentiations, and as we have seen in the chapter on cleavage, 

 it is just as easy to draw homologies between cell groups in the 

 blastula stage, or in an earlier cleavage stage, as it is between 

 the later appearing germ layers. It is not too much to say 

 that in some cases homologies may be drawn between various 

 formed substances in the undivided egg. Animal and vegetal 

 poles of the ovum, cleavage patterns, cell-groups, micromeres, 

 macromeres, upper and lower poles of the blastula, are all 

 constant and comparable features of development no less than 

 inner, outer and middle germ layers. We may recall that the 

 cell known as 4d may be identified and its history and fate 

 compared, in the cleavage of many groups, even in different 

 phyla. This cell whose form, position, and derivation are so 

 constant, may or may not form "mesoderm"; even when it 

 does form mesoderm this may go to form very different parts of 

 the embryonic and adult structure. Often the " mesoderm" 

 may be a cell, just as truly as a layer. 



Summarizing we may say that while the arrangement of the 

 cells of the embryo in the form of definite layers is almost 

 universal, at the same time, in the comparison of different 

 groups or of different modes of development, these layers 

 exhibit great inconstancy in their relations to one another, 

 and to the structures forming them and formed from them. 

 The germ layers are valuable, indeed indispensable descriptive 

 units, but they do not represent primary differentiations, and 

 their homologies are no more, though probably no less, funda- 

 mental throughout groups larger than phyla, than are many 

 other structures of the developing organism. 



MORPHOGENETIC PROCESSES 



It remains now to describe some of the more general processes by 

 which the rudiments of the organs and tissues of the embryo may be 

 formed out of the layers or cell masses of the gastrula and post-gastrula 

 stages. We shall not attempt to describe here the actual formation of 

 any specific embryonic structure, but rather shall give a brief classifica- 



