INTRODUCTION 13 



In the maturation process, two kinds of chromosomes are usu- 

 ally distinguishable. These are the ordinary chromosomes, 

 which are designated as the autosomes, and others which differ 

 from them both in appearance and in behavior and are termed 

 the sex-chromosomes or heterosomes. 



In some instances, the immature germ cells of each sex have 

 a pair of sex-chromosomes. Typically, the sex-chromosomes of 

 the female are both alike and are called the X-chromosomes. 

 On the other hand, in the immature germ cells of the male one 

 of the sex-chromosomes is frequently smaller than the other 

 and is designated as the Y-chromosome. Under these conditions 

 the autosomes of the maturing male germ cells are distributed 

 evenly and equally among the mature cells, but the sex-chromo- 

 somes become segregated so that any individual cell in addition 

 to its autosomes contains either an X- or a Y-chromosome but 

 never both. In the male, then, half of the sperm cells contain 

 the Y-chromosome but no X, while the other half contain an 

 X-chromosome but no Y. Every mature egg cell contains the 

 autosomes and a single X-chromosome. Fertilization of an egg 

 cell by a spermatozoon containing the Y-chromosome produces a 

 zygote from which only a male could develop, but fertilization 

 by a spermatozoon with an X-chromosome produces a zygote 

 from which only a female could develop. 



The Y-chromosome may be entirely lacking in some species. 

 Under these conditions, there are two kinds of male sex cells 

 formed: one with the autosomes plus an X-chromosome, the 

 other with autosomes alone. 



While the dimorphism in the germ cells is usually characteristic 

 of the male, there are some instances recorded, as in the birds, 

 in which all of the male germ cells are alike, but the female germ 

 cells show chromosome differences. 



Composition and Cleavage of Zygote. — Except for the possi- 

 bility that one of the fusing germ cells may have one more 

 chromosome than the other, the two gametes contribute equally 

 to the chromatin content of the zygote. The cytoplasm of the 

 zygote is that contained in the mature egg except for a practically 

 insignificant amount brought in by the entering sperm cell. The 

 stored food material, which provides the energy requisite to the 

 life processes of the embryo, is furnished by the deutoplasm of 

 the egg cell, except in those forms which have accessory yolk 

 cells accompanying the egg and those which receive nourishment 



