204 BIOLOGY: GENERAL AND MEDICAL 



In vegetables it cannot be shown that the germinal 

 cells differ essentially from the somatic cells. They 

 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 and 

 somatic cells possess the same number of chromosomes, 

 yet the gametes contain but half as many. This depends 

 upon a "reduction of chromosomes" discovered by van 

 'Beneden, and seen in the maturation of the germinal 

 cells. It is a matter of much interest, and, as it has 

 fundamental bearing upon the problems of inheritance 

 and variation, deserves much attention. 



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 awakens them to ac- 

 tivity. 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 apart 

 in the embryonal gonad testis multiply slowly during 

 the period of growth and development, and perhaps more 

 rapidly during the period of sexual activity. No essential 

 difference in appearance separates them from the somatic 





