262 



INSECTA. 



T ~ 



composecl principally of food-yolk (do). The peripheral layer of 

 protoplasm, the periplasm or perivitellus ("Keimhautblasteni" of 

 Weismann, No. 87), has only been found wanting in a few cases,. 



but it is usually quite thin and incon- 

 siderable, and, as compared with the 

 amount of the central food-yolk, it 

 might almost be thought to be dis- 

 appearing. Only the eggs of certain 

 small Insects are found to be com- 

 paratively poor in yolk. Some of 

 these are forms in which the larva 

 which emerges from the egg is dis- 

 tinguished by its small size (oviparous 

 Aphidae), or in which the nourishment 

 of the embryo is provided for in some 

 other way, either by its development 

 within the body of the mother (vivi- 

 parous Ajriiidae), or by the embryo 

 passing through its development endo- 

 parasitically in the coelomic fluid of 

 some other Insect (Ichneumonidae). In 

 all these forms the poverty in yolk has 

 a determining influence on the course 

 of embryonic development as will be 

 seen later. These modifications are 

 probably secondary, and an ovum well 

 provided with food-yolk is no doubt 

 to be regarded as the primitive type 

 of Insect egg. 



The central yolk-mass in the Insect 

 egg (do) consists of a fine network 

 of formative protoplasm, within the 

 meshes of which the numerous particles 

 of food-yolk and spherical fat-drops are 

 contained. The elements of the food- 

 yolk appear as strongly refractive bodies 

 which are spherical or polygon ally flat- 

 tened by mutual pressure, and appar- 

 ently structureless and homogeneous. 

 The germ-vesicle of the maturing insect egg lies in the central 

 part of the yolk, and appears as a large vesicular nucleus provided 



Fni. 129. — Diagrammatic median sec- 

 tion through the egg of Musca at 

 the stage of fertilisation (taken 

 from drawings by Henking and 

 Blochmann). ch, chorion ; d, dor- 

 sal side of the egg ; dh, vitelline 

 membrane ; do, food-yolk ; g, glu- 

 i mi 'us cap over the micropyle; k, 

 peripheral protoplasm (periplasm 

 or perivitellus); m, micropyle; p, 

 male and female pronucleus before 

 fusion ; r, polar bodies ; v, ventral 

 side of the egg. 



