SPERMATOGENESIS OF THE HORSE. 30! 



in the fact that the resulting nine chromosomes are not one half 

 the size of the original eighteen chromosomes of these cells, 

 but exactly of the same size and apparently quadrivalent. This 

 quadrivalent nature becomes obviated after the division of the 

 secondary spermatocyte, where the resulting chromosomes are 

 bivalent. The primary spermtocyte division is undoubtedly 

 the reduction division and, speaking in terms of univalence, one 

 of the resulting secondary spermatocytes receives eighteen chro- 

 mosomes and the other eighteen plus the accessory. In terms 

 of bivalence the one type of secondary spermatocytes receive 

 nine chromosomes and the other nine plus the accessory (Figs. 



30-33)- 



VI. SECONDARY SPERMATOCYTE. 



No resting stage occurs in the secondary spermatocyte, a 

 condition similar to that frequently found in the spermatogenesis 

 of the pig. The second pairing of the chromosomes also takes 

 place here as it does in the pig (Wodsedalek, '13), man (Guyer, 

 '10), and opossum (Jordan, 'n). In the pig, however, this 

 pairing takes place much later, never before the cell is com- 

 pletely divided. The secondary spermatocytes divide soon after 

 they are formed and not infrequently the spindles are formed 

 in the two cells resulting from the first spermatocyte division 

 while they are still in close contact. Nine chromosomes arrange 

 themselves in the equatorial plate for division in the one type 

 of secondary spermatocyte (Figs. 42-45), and nine plus the 

 accessory in the other (Figs. 34-38). All of the chromosomes, 

 including the accessory when it is present, divide in these cells 

 (Figs. 36-47). The accessory usually lies a little to one side of 

 the other chromosomes (Figs. 34 and 35), and again, as in the 

 spermatogonia, divides a little in advance of the other chromo- 

 somes (Figs. 36-38). This may be due to the partial separation 

 of the two halves of this body even long before the other chromo- 

 somes line up for division in this stage (Figs. 21-33). The heart- 

 shape it assumes during the later stages of the primary spermato- 

 cyte division and retains during the secondary spermatocyte, is 

 no doubt due to a partial separation at one end of the two com- 

 ponents. The chromatoid body remains very conspicuous (Figs. 

 35-55). 



