ONTOGENESIS 205 



deuteroplasm is intended to nourish the developing 

 embryo, hence the magnitude of the yolk must bear 

 some reference to the dependence of the embryo upon 

 that form of nourishment. In cases in which the egg is 

 quickly developed into a self-sustaining larva, there is 

 no yolk; in cases where it becomes attached to the 

 uterine wall of the parent from whom it derives nourish- 

 ment, the yolk may be inconspicuous, but in those cases 

 the birds, reptiles, fishes- where the egg is entirely 

 separated from the parent and completes its embryonal 

 transformations without a larval form in which addi- 

 tional nourishment can be secured from without, the 

 yolk must be large enough to supply all of the embryonal 

 requirements. 



The encumbrance of the yolk modifies the earliest 

 transformations of the egg cleavage as will be shown. 



Before leaving the eggs it is necessary to give brief 

 attention to the subject of fertilization. When an egg 

 is surrounded by a leathery or calcareous covering before 

 expulsion from the maternal body, it cannot be subse- 

 quently fertilized, so that in such cases the spermatozoa 

 must have been emitted into the maternal organs, where 

 they meet and fertilize the egg before its final coverings 

 are provided, unless such coverings contain one or more 

 openings micropylse for the special purpose of admit- 

 ting the spermatozoa. 



The developmental process begins by cell division, 

 "cleavage, " or "segmentation" of the ovum, which is 

 followed by that cellular multiplication through the 

 continuance of which the different tissues and organs 

 are produced. 



The mode of segmentation differs in different eggs, 

 partly through peculiarities of the eggs, partly through 

 inherited impulses. Hertwig presents the following 

 scheme of cleavage: 



I. Type. Holoblastic eggs (without yolks). 



Total cleavage: a. Equal cleavage (lower inverte- 

 brates and mammals), b. Unequal cleavage (mol- 

 lusk and amphibia). 



