INTRODUCTION. 



Since the ovum in its primitive form as above described 

 represents a single cell, we may speak of the vitellus 1 as the 

 protoplasm of the egg-cell, the germinal vesicle as its nucleus, 

 and the germinal spot as its nucleolus. An outer limiting 

 membrane, corresponding to the vitelline membrane, is not an 



.AT 



FIG. 1. DIAGRAM OF THE UNIMPBEGNATSD OVUM. 

 I), vitellus ; KB, germinal vesicle ; KF t germinal spot. 



integral part of the cell, but may be developed as a hardening of 

 the peripheral protoplasm, consequent on a process of differen- 

 tiation. 



In all Vertebrates, the contact of the generative products of the 

 male, the sperm-cells (spermatozoa) is an absolute necessity for 

 the development of the ova. A spermatozoon makes its way into 

 the interior of the ovum, and a portion of it finally unites in a 

 definite manner with the modified germinal vesicle to form a single 

 body the first segmentation nucleus. 



This modification of the germinal vesicle takes place as follows. Before 

 fertilisation occurs, two polar cells 2 are constricted off from the ovum, part 

 of the germinal vesicle passing into each, and the remainder being spoken of 

 as the female pronucleus. The polar bodies are given off at different 

 times in different animals : they may be formed while the ovum is still within 

 the ovary, or, on the other hand, they may arise at the time of fertilisation. 





1 The vitellus consists of two different substances protoplasm and deutero- 

 plasm (yolk) in varying proportions in different animals. 



2 The two most important views as to the meaning of the polar cells are those 

 of (1) Balfourand van Beneden, and (2) Weismann. The first-named authors suppose 

 that the egg, being a product of both sexes, is primitively hermaphrodite. By the 

 extrusion of the polar bodies, the male portion of the egg is thrown out, and the 

 remainder thus becomes unisexual (female), and ready for the entrance of the 

 spermatozoon. This process would thus be a contrivance for the prevention of 

 parthenogenesis. 



Weismann distinguishes in every animal body two kinds of cells, somatic and 

 generative cells. As all the cells arise as products of the segmentation of the 

 ovum, they are originally quite similar morphologically, and each would thus consist 

 of a "somatic" and of a "generative" portion. In order that certain of them 

 should give rise to definite generative cells, it is necessary that the somatic portion 

 should be got rid of, and this is effected by the extrusion of the polar bodies. 



The first hypothesis -presupposes that in parthenogenesis no polar bodies are 

 formed. Weismann has lately, however, proved their existence in the partheno- 

 genetic summer eggs of Daphnidse, and this view is consequently rendered impro- 

 bable. In the development of the male generative cells, a certain portion of each 

 primitive seminal cell also remains passive, not giving rise to spermatozoa. 



B 2 



