FERTILIZATION OF THE EGG 525 



Meloe is a parasitic form, and probably only a few of its many offspring ever 

 succeed in gaining access to the egg of the bee." 



In the eggs of Chrysopa the yolk-granules are remarkably small, 

 so that the primitive band is in strong contrast to the yolk in dolor 

 and density. When crushed, the yolk does not flow out as a liquid, 

 but in a pasty mass, and we have questioned whether, as in the eggs 

 of Limulus, whose yolk is solid with fine granules, the denseness of 

 the yolk is not connected in the way of cause and effect with their 

 exposed situation. 



The central or yolk-mass (Fig. 502, do) consists chiefly of rounded 

 masses of yolk, with fat-globules, between which extends a fine net- 

 work of protoplasm. 



The elements of the yolk are spherical and strongly refractive, by 

 pressure becoming polygonal structureless homogeneous bodies. 



The germinal vesicle of the ripe insect-egg lies in the centre of the 

 yolk, where it appears as a large vesicle-like cell-nucleus containing 

 a few chromatin elements. 



'-. Maturation or ripening of the egg 



Before the eggs of animals can be fertilized, they require in some 

 observed cases, and probably in animals in general, to undergo a 

 series of changes, which, as observed in the starfish, etc., consists in 

 the replacement of the germinal vesicle by a very much smaller egg- 

 nucleus, and also at the same time the construction at one pole of the 

 egg of the directive or polar bodies (Fig. 502, ?). Towards the end 

 of the ripening process of the insect egg this vesicle, according to 

 Blochmann, passes to the dorsal surface of the egg, and is trans- 

 formed into the directive spindles (Richtungspindefy . 



c. Fertilization of the egg 



The egg next requires the penetration and admission into the 

 yolk-interior of a spermatozoon. 



This process is essentially in insects, as in other animals, the 

 fusion of the sperm-nucleus with the nucleus of the egg. Under 

 normal conditions but a single spermatozoon is required for fertiliza- 

 tion. As shown by Hertwig, in the sea-urchin, after the spermato- 

 zoon has penetrated into the egg, the head, and the small rounded 

 body, called a centrosome, can still be recognized, but the tail becomes 

 fused with the yolk of the egg. In the protoplasm of the egg (called 

 cytoplasm) the achromatic end of the sperm-nucleus gives rise to 



