vn] VERSON'S CELL; EGG STRUCTURE 105 



giving rise to a large central mass surrounded by a layer of 

 smaller spermatogonia, and later of spermatocytes and 

 spermatids. The cytophore itself is non-nucleated, and 

 seems to serve both for the supply of nourishment and for 

 a point of attachment for the spermatozoa until they are 

 mature. This development of spermatozoa by means of a 

 cytophore is well known in the earthworm, in which the 

 primitive germ-cells are discharged from the testis at an 

 early stage, and the whole of the later spermatogenesis takes 

 place in the seminal vesicles. 



2. OOGENESIS 



The development of the egg in different animals is much 

 less uniform than that of the spermatozoon, in consequence 

 of their relatively great variability in size, structure and 

 amount of yolk in different eggs, and only an illustrative 

 account of some typical features can be given here. An egg 

 before fertilisation is usually a moderately or very large 

 cell containing a variable amount of yolk and enclosed in 

 some kind of membrane. The nucleus before the maturation 

 divisions is commonly large and vesicular, and nearly 

 always has a conspicuous nucleolus; it may be situated 

 either near the centre of the egg, or, especially when there 

 is much yolk, near the surface in a patch of protoplasm free 

 from yolk granules. The egg is often enclosed in a very 

 delicate vitelline membrane, though it may be naked, and 

 outside this there is frequently some sort of outer covering 

 or shell (e.g. the chorion of insects or zona radiata of verte- 

 brates), which may belong to the egg itself or be produced 

 by the surrounding follicle cells. When this outer covering 

 is present before fertilisation, it is usually pierced by a 

 micropyle, a fine canal, at the pole of the egg nearest to the 

 nucleus, by which the spermatozoon enters. Not infrequently 



