THE ULTIMATE SEX-ELEMENTS. 



77 



CHAPTER VII. 



THE ULTIMATE SEX-ELEMENTS. 



{General and Historical?) 



tn our analysis of sex-characters we have followed the general course 

 of biological history. We first passed from the form and habit 

 of a male or female organism to the structure and functions of 

 the sexual organs. In discussing hermaphroditism, we had occa- 

 sion to refer to a third step of biological analysis — that which^ 

 involves an investigation of the properties of the tissues. Now it is 



In so 

 form 



in 



Fig. 18. — Mammalian ovum, showing nucleolus (a), nucleus ([>), 

 yelk (c), external porous zone or zona pellucida (d), and follicular 

 cells (e). — From Hertwig, after Waldeyer. 



necessary to penetrate deeper, namely, to the sex-cells. After these 

 have been considered, not in themselves, but finally and fundamentally 

 in terms of the changes in the protoplasm that make them what they 

 are, then we shall be in a better position to reascend to some of the 

 problems of reproduction. 



I. The Ovum-theory. — It is now a commonplace of observa- 

 tion and established fact that all organisms, reproduced in the ordinary 

 way, start in life as single cells. We see insects laying their ova upon 

 plants, or fishes shedding them in the water, and may watch how 

 these cells, provided they be fertilized, give rise eventually to the 



