228 EVOLUTION AND ANIMAL LIFE 



(c) ; each of these divides, and this division is repeated until a 

 greater or lesser number (varying with the various species or 

 groups of animals) of cells is produced. These cells may not 

 all be of the same size, but in many cases they are, no struc- 

 tural differentiation whatever being apparent among them. 



The phenomenon of repeated division of the germ cell is 

 called cleavage, and this cleavage is the first stage of develop- 

 ment in the case of all many-celled animals. The germ or embryo 

 in some animals consists now of a mass of few 7 or many undif- 

 ferentiated primitive cells lying together and usually forming a 

 sphere (Fig. 134 ; c), or perhaps separated and scattered through 

 the food yolk of the egg. The next stage of development is 

 this: the cleavage cells arrange themselves so as to form a 

 usually hollow sphere or ball, the cells lying side by side to 

 form the outer circumferential wall of this hollow sphere (/). 

 This is called the blastula or blastoderm stage of development, 

 and the embryo itself is called the blastula or blastoderm. 

 This stage also is common to all the many-celled animals. 

 The next stage in embryonic development is formed by the 

 bending inward of a part of the blastoderm cell layer, as shown 

 in (g) (or the splitting off inwardly of cells from a special part 

 of the blastula cell layer). This bending in may produce a 

 small depression or groove; but whatever the shape or extent 

 of the sunken-in part of the blastoderm, it results in distinguish- 

 ing the blastoderm layer into two parts, a sunken-in or inner 

 portion called the endobhist and the other unmodified portion 

 called the ectoblast. Endo- means within, and the cells of the 

 endoblast often push so far into the original blastoderm cavity 

 as to come into contact with the cells of the ectoblast and 

 thus obliterate this cavity (h). This third well-marked stage 

 in the embryonic development is called the gastrula stage, and 

 it also occurs in the development of all or nearly all many- 

 celled animals. 



In the case of a few of the simple many-celled animals the 

 embryo hatches that is, issues from the egg at the time of 

 or very soon after reaching the gastrula stage. In the higher 

 animals, however, development goes on within the egg or 

 within the body of the mother until the embryo becomes a 

 complex body, composed of many various tissues and organs. 

 Almost all the development may take place within the egg, 

 so that when the young animal hatches there is necessary 



