258 ANIMAL STUDIES 



ber (varying with the various species or groups of animals) 

 of cells is produced (Fig. 143, d). The phenomenon of 

 repeated division of the germ cell is called cleavage, and 

 this cleavage is the first stage of development in the case 

 of all many-celled animals. The first division of the germ 

 cell produces two equal cells, but in some of the later 

 divisions the new cells formed may not be equal. In some 

 animals all the cleavage cells are of equal size ; in some 

 there are two sizes of cells. The germ or embryo animal 

 consists now of a mass of few or many undifferentiated 

 primitive cells lying together and usually forming a sphere 

 (Fig. 143, e), 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 hollow sphere or ball, the cells lying side by side to form 

 the outer circumferential wall of this hollow sphere (Fig. 

 143, /). This is called the Uastula 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 blasto- 

 derm cell layer, as shown in Fig. 143, g. 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 distinguishing the blastoderm layer into two 

 parts, a sunken-in portion called the endoblast and the 

 other unmodified portion called the ectoblast. Endo- means 

 within, and the cells of the endoblast usually push so far 

 into the original blastoderm cavity as to come into contact 

 with the cells of the ectoblast and thus obliterate this cavity 

 (Fig. 143, 7^). This third well-marked stage in the embry- 

 onic development is called the gastrula * stage, and it also 



* This gastrula stage is not always formed by a bending in or in- 

 vagination of the blastoderm, but in some animals is formed by the 

 splitting off or delamination of cells from a definite limited region of 



