192 BIOLOGY: GENERAL AND MEDICAL 



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 

 cells. They possess the same number of chromosomes 

 and divide after the usual karyokinetic changes homo- 

 type mitosis. 



When the time of functional activity arrives these cells, 

 which in the higher animals are known as primary 

 spermatocytes, manifest certain peculiar proliferative 

 activities, the chief of which is known as the reduction 

 division. The chromatic substance in the nucleus gathers 

 together to form the usual number of chromosomes, but in- 

 stead of assuming their customary appearance and arrange- 

 ment they appear in pairs or gemmini. This conjugation 

 of the chromosomes leads to an appearance easily misin- 

 terpreted to mean that the cell has either twice the usual 



