190 BIOLOGY: GENERAL AND MEDICAL 



and may progress by the usual mode of cell division, 

 and no other difference between dividing germinal cells 

 and dividing somatic cells can be detected than is indi- 

 cated by the number of chromosomes which is twice 

 as great in the germinal as in the somatic cells. During 

 this time the chromosomes are filamentous in appearance, 

 and the division of a cell results in the passage into each 

 half of an equal quantity of chromatin. In this manner 

 complete quantitative and qualitative uniformity of 

 the germinal cells is maintained. 



When the time for the maturation of the spermatocyte 

 has arrived, a change in the shape and appearance of 

 the chromosomes is observed. Instead of maintaining 

 their filamentous character, they now become globular 

 and peculiarly arranged. Thus, supposing the somatic 

 cells of the organism under study to be provided with 

 sixteen filamentous chromosomes, as in man, we find 

 the spermatocytes at this stage furnished with thirty-two, 

 of spherical shape, peculiarly grouped into eight tetrads. 



This new arrangement of things having been effected, 

 the cell, which is known as a primary spermatocyte, 

 divides into two in such manner that each tetrad 

 divides into two dyads and each resulting cell possesses 

 sixteen chromosomes in pairs or dyads. This is no 

 sooner ended than each cell or secondary spermatocyte 

 prepares for another division without other nuclear 

 change than an alteration in the position of the dyads, 

 each of which separates into two monads, so that each 

 of the four cells finally descending from the primary 

 spermatocyte is provided with but eight chromosomes 

 of spherical shape. These cells are the future sper- 

 matozoids, whose further morphological transformations 

 have nothing to do with the chromosomes and need not 

 further concern us at present. 



Oogenesis. This process corresponds with spermato- 

 genesis with the single exception that, instead of resulting 

 in four similar and functionally active cells, it usually 

 terminates in four cells, of which one is functional and 



