258 ANIMAL STUDIES 



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

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

 peated division of the germ cell, and usually the surround- 

 ing yolk, 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 usually 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 scat- 

 tered through the food yolk of the egg. The next stage of de- 

 velopment 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 endoUast and the 

 other unmodified portion called the ectoUast. 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 

 (Fig. 143, Ji). 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 



