ONTOGENESIS 221 



same, but the size varies, the animal cells being smaller 

 than the vegetative cells. Equality of numbers is not, 

 however, preserved, for either the richness of protoplasm 

 in the animal half or the magnitude of the cells of the 

 vegetative half determines that the former outgrows 



SECTION OF MORULA. 



SECTION OF BLASTULA. 



Archiccele. 



a b 



FIG. 84. a, Section of morula; 6, section of blastula. (Masterman.) 



the latter until with 128 cells in the animal half there 

 may be but thirty-two in the vegetative half, and so on. 



The inequality of the cleavage is in direct proportion 

 to the quantity of yolk at the vegetative pole. Thus, 

 in holoblastic eggs, the cleavage may be equal; in the 

 enormous meroblastic eggs of birds the yolkless proto- 

 plasm assembled at the animal pole is alone able to un- 

 dergo segmentation, and the primitive assemblage of cells 

 resulting from this localized cleavage appears as a cellu- 

 lar disc floating upon the surface of the yolk. This 

 germinal disc receives the name "blastoderm." 



The segmentation first results in the formation of a 

 solid cellular mass which bears a partial resemblance 

 to a mulberry and is known as a morula. Every egg 

 passes through this stage. Soon, however, the morula 

 becomes changed by assumption of fluid or by vacuo- 

 lization of the inner cells, and a hollow sphere is formed, 

 the blastula, surrounded on all sides by a single layer of 

 blastomeres. 



The eggs of the invertebrates always form blastulae, 

 some of which are ciliated, free-swimming, and self- 

 sustaining larval forms. Such are called monoblastic 

 larva. They are typically centro-symmetric, the hollow 

 centre being known as the archicele or blastocele, the 

 cellular layer as the archiblast. The sponges have 

 larvae of this form. 



