668 TISSUES OF REPRODUCTION. [BOOK iv. 



In the higher animals the reproduction of the whole individual 

 can be effected in no other way than by the process of sexual 

 generation, through which the female representative element or 

 ovum is, under the influence of the male representative or sperma- 

 tozoon, developed into an adult individual. 



We do not purpose to enter here into any of the morphological 

 problems connected with the series of changes through which the 

 ovum becomes the adult being; or into the obscure biological 

 inquiry as to how the simple all but structureless ovum contains 

 within itself, in potentiality, all its future developments, and as to 

 what is the essential nature of the male action. These problems 

 and questions are fully discussed elsewhere ; they do not properly 

 enter into a work on physiology, except under the view that all 

 biological problems are, when pushed far enough, physiological 

 problems. We shall limit ourselves to a brief survey of the more 

 important physiological phenomena attendant on the impregnation 

 of the ovum, and on the nutrition and birth of the embryo. 



