REPRODUCTION 189 



appear only when the reproductive process is anticipated, 

 attain to the necessary degree of specialization in a few 

 generations, are characterized by a preparation for 

 fertilization to all intents and purposes identical with 

 that of the animal cells, and then having been fertilized 

 by conjugation with another specially adapted cell, lose 

 the reproductive quality and become vegetative in 

 character once more. Thus, in vegetables the repro- 

 ductive activity may be said to pervade the cells gener- 

 ally, while in animals it is more and more restricted to 

 the few cells comprising the germ plasm. 



It is of the utmost importance that the steps pre- 

 liminary to sexual fertilization be carefully followed, 

 and for this purpose it will be necessary once more to 

 enter the domain of cytology. 



Among both plants and animals the germinal differ 

 from the somatic cells in possessing twice the number 

 of chromosomes. Yet the gametes contain but half as 

 many. This depends upon a " reduction of chromosomes" 

 seen in the maturation of the germinal cells. It is a 

 matter of much interest, and as it has fundamental bear- 

 ing upon the problems of inheritance and variation, 

 some attention must be devoted to the histogenesis of 

 the gametes. 



Any of the higher plants or animals will be found to 

 possess specialized germinal cells set aside in the gonads 

 or sex organs until sexual maturity awakes them to 

 activity. As there are two sexes, and the sex organs 

 and their products differ, two kinds of gametes, the 

 male, or spermatozoa, and the female, or ova, are to be 

 studied, and two subjects, spermatogenesis and oogenesis, 

 appear for investigation. 



Spermatogenesis. The germinal cells early set aside 

 in the animal gonads are few and therefore undergo great 

 increase in number through karyokinetic division before 

 enough shall have been formed to supply the require- 

 ments of sexual life. Even during the period of sexual 

 activity multiplication of these cells may be required 



