ONTOGENESIS 199 



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 micropylae 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 inherent in them. 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- 

 lusks and amphibia). 

 II. Type. Meroblastic eggs (with yolks). 



Partial cleavage: a. Discoidal cleavage (fishes, 

 birds, and reptiles), b. Superficial cleavage (in- 

 sects and arthropods). 



Thus it is at once apparent that the presence or ab- 

 sence of a considerable yolk determines whether the 

 cleavage shall be total or partial, and the examination 

 of any thorough description of the development of the 

 chick will make clear the manner in which the enormous 

 yolk modifies segmentation. 



