THE ANIMAL BODY 125 



It is important in this connection to understand how these 

 modifications are related to the third primary germ layer, or 

 mesoderm, which, as we have stated, plays a part in the 

 development of all forms above Hydra. For the sake of con- 

 creteness we shall describe the development of the Earth- 

 worm from the fertilized egg to the establishment of the general 

 body plan, though it must be borne in mind that in no two 

 species of animals is the process of development identical. 



After fertilization, the egg of the worm proceeds to divide 

 first into two cells, then four cells, eight cells, and so on, with 

 more or less regularity, until a condition is attained in which 

 many relatively small cells are arranged about a central 

 cavity. This stage of the embryo will be recognized as the 

 blastula. (Fig. 69.) 



The various cells of the blastula appear essentially the 

 same except that those at one end are somewhat larger than 

 at the other. The larger cells now sink into and nearly ob- 

 literate the central cavity of the blastula, thus forming a 

 typical gastrula stage composed of two layers of cells, ec- 

 toderm on the outside and endoderm on the inside. The 

 infolded endoderm pouch (ENTERIC POUCH) enclosing the en- 

 teric cavity eventually becomes the main part of the alimen- 

 tary canal of the worm, its present opening to the exterior 

 (BLASTOPORE) forming the mouth. The ectoderm is destined 

 to form the skin, or outer layer of the worm's body. 



While these two primary germ layers are being established, 

 the developing embryo shows the rudiments of the third 

 primary germ layer (mesoderm) in the form of two cells 

 (POLE CELLS) which leave their original position in the wall 

 of the embryo and take up a place between the ectoderm 

 and endoderm ; that is, in the remnant of the cavity of the 

 blastula which the invagination process during gastrulation 

 has not completely obliterated. Here the pole cells, by di- 



