256 



GENERAL EMBRYOLOGY 



bear to the embryo or later organism. As distinct morpho- 

 logical and physiological units they represent real differentia- 

 tions at a very early stage of development, and may truly be 

 said to form embryonic rudiments of structures appearing later 

 in the form of germ layers or derivatives of these. 



FIG. 122. Diagrams illustrating the value of the quartets in three animals. 

 From Wilson, " Cell." Ectoplasm is unshaded; mesoplasm is dotted; endoplasm 

 is vertically ruled. A. The Polyclad, Leptoplana, showing mesoplasm formation 

 in second quartet. (Compare Planocera, Fig. 120, where Surface finds meso- 

 plasm in cell 4d descendants.) B. The Gasteropod, Crepidula. C. The 

 Pelecypod, Unio. 



Not only this but comparison of the cell lineages of different 

 classes and phyla often brings out the fact that particular 

 cells can be identified and compared in diverse groups of ani- 

 mals, making it possible to apply the idea of homology to blas- 

 tomeres and groups of blastomeres in the early embryo, as well 



