MORPHOLOGY OF ANIMALS 



( 1 ) The Fertilized Egg which is a single cell, comparable 

 to a one-celled organism; (2) the Cleavage Stages, during 

 which the egg divides into a large number of cells, which are 

 roughly comparable to a protozoan colony (Fig. 1, A-D) ; 

 (3) the Blastula, which is typically a hollow sphere, radially 

 symmetrical, composed of many cells, surrounding a central 

 cleavage cavity, and resembling somewhat a Volvox colony 

 (Fig. 1, E, F) ; (4) the Gastrula (Fig. 1, G-I), which is 

 typically composed of two primary layers of cells, the outer 

 layer, the ectoder?n {ec), covering the body, the inner, the 

 endoderm (en)^ lining the digestive cavity or enteron {gas- 

 trocoel, g), which opens to the exterior through a primary 

 mouth or blastopore {bp), A third layer or group of cells, the 

 mesoderm, arises from one or both of the primary layers or 

 from certain of the cleavage cells before layers are formed, 

 and comes to lie between ectoderm and endoderm. It may 

 consist of loose, scattered cells {mesenchyme, ms), or of an 

 epithelial layer {mesothelium, me), or of both (Fig. 1, 

 H,I). 



Such a gastrula is a real metazoan and some of the lowest 

 metazoa are, in their adult condition, little more than gastru- 

 lae (e.g. Hydra). The gastrula is the ground-form of all 

 metazoa and from this stage onward in the development of 

 higher metazoa different phyla diverge, the gastrula under- 

 going certain modifications and complications which trans- 

 form it into the adult form characteristic of each phylum. 

 The earliest stages of ontogeny are in many respects like the 

 lowest living organisms, as shown in the following table : 



c 143 



