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.
b. 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, r). 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 transformed into the directive spindles (Richtungspindel).
c. Fertilization of the egg
The egg next requires the penetration and admission into the yolk-interior of a spermatozoön.
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 spermatozoön is required for fertilization. As shown by Hertwig, in the sea-urchin, after the spermatozoön 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 conspicuous rays, like those observed in ordinary cell-division. Preceded by these rays, the sperm-nucleus or male pronucleus (Fig. 502, p) moves towards the nucleus of the egg, and finally fuses with it, thus forming a new single nucleus. This latter, which is called “the cleavage nucleus,” rapidly forms a nuclear or “cleavage spindle” (Fig. 503). This act gives an impulse to the cleavage of the egg, which is the first step in the formation of the embryo. All these changes have yet to be worked out in detail in insects by microscopic sections of the egg, whose generally hard and opaque egg-shells present great obstacles to such work.
d. Division and formation of the blastoderm[[80]]
In insects as in most other Arthropoda the segmentation of the yolk is superficial and not total. The ovum is centrolicithal, i.e. the yolk is concentrated at the centre of the egg, and surrounded by a peripheral layer of transparent protoplasm (the Keimhautblastem).
