CHAPTER III 



THE EGG, FERTILIZATION, MATURATION, AND CLEAVAGE 

 THE EGG OF THE EUTRACHEATA 



The germ cells are differentiated at an early period, in some cases 

 before the formation of the blastoderm. Dividing into two groups, they 

 migrate forward and become lodged on or near the genital ridges which 

 have meanwhile developed on the splanchnic mesoderm. The genital 

 ridges give rise to the gonads, the embryonic reproductive organs, from 

 which the testes or ovaries later are formed. Within these organs the 

 germ cells develop into eggs or sperms during postembryonic life. 



Except in some parasitic forms the egg is outwardly protected by a 

 firm shell, or chorion, in which there are one or more orifices for the 

 passage of the sperms. Immediately within the chorion is the thin, 

 noncellular vitelline membrane. The contents of the egg are composed 

 chiefly of two elements : protoplasm and deutoplasm. The former is the 

 formative yolk; the latter, the food material upon which the developing 

 embryo will live. The formative protoplasm (ooplasm) usually consists 

 of a thin peripheral layer, or periplasm (Keimhautblastem) , immediately 

 under the vitelline membrane; and an inner reticulum, or net, in the 

 meshes of which the deutoplasm, or food yolk, is held. The periplasm 

 seems to be lacking in Orthoptera and some Hymenoptera. At or near 

 the center is the nucleus surrounded by a protoplasmic envelope (nuclear 

 cytoplasm). 



The chorion, or eggshell, is formed by the follicular cells of the ovarioles 

 of the parent in a manner similar to the formation of the cuticula by the 

 epidermis of the body wall. The chorion, however, is not chitinous, as 

 was formerly believed, but differs in containing sulphur, in having a 

 higher nitrogen content, and in being less resistant to alkalines. Kor- 

 schelt (1887) states that when first formed it is soft and plastic and that 

 it adjusts itself to the changing form of the developing egg. The chorion 

 does not always develop simultaneously over the egg but may form first 

 at the posterior end, gradually covering the surface. The reticulate or 

 otherwise complicatedly marked egg surface is produced by the impres- 

 sion of the ends of the ovarian follicle cells upon the plastic chorion. The 

 coriaceous substance of which the chorion is composed and to which the 

 term "chorionin" has been applied is secreted not only by the ends of 

 the follicular cells but also by the sides from between adjacent cells 

 resulting in the strange markings characteristic of certain insect eggs. 



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