194 ESSENTIALS OF BIOLOGY 



of the distortions introduced into segmentation by the presence 

 of food-yolk in the egg (Fig. 27, 4). But it can very generally 

 be recognized and we see in it a general, animal, developmental 

 phase in embryogeny. 



In the above figures the further development of the gastrula 

 is represented in 6, a polychaete worm and in 7, the blastoderm, 

 or formative membrane of the chick embryo. 



The germ-layers. Thus are laid down three separate sheets of 

 cells, or cell-layers — the ectoderm (or outer layer) ; the endoderm 

 (or inner layer) and the mesoderm (or middle layer). There are 

 cases of development in which these germ-layers are difficult to 

 recognize : nevertheless, they are so often present in typical form, 

 and they are easily recognizable in so many different kinds of 

 animals that we may regard them as definitely tectonic arrange- 

 ments of the cells of the embryo anticipatory of the further forma- 

 tions of the organ-anlagen. When they become established an 

 important cavity appears in the embryo : this is the coelom (see 

 the above figure) and it is typically a cleft bounded by two layers 

 of mesoderm, or it may be an irregular cavity, or one or more 

 definite vesicles with mesodermic walls. 



706. The Potencies and Fates of the Germ-layers and 

 Cavities. The germ-layers and cavities are the foundations of 

 the subsequent tectonics in the development. In general these 

 are the potencies of the various parts : we refer to normal and 

 typical embryogenies, for there are many exceptions to the 

 generalization. 



That is, by the divisions and subsequent cell-assemblies of the 

 cells of each germ-layer the organs are formed, as in the above 

 general scheme. 



