STUDIES ON THE CELLS OF CATTLE. 



3. Secondary Spermatocytes. 



No resting stage occurs in the secondary spermatocyte. This 

 condition also occurs in the horse and is frequently found in the 

 pig, according to my former studies. The secondary spermato- 

 cytes 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 (Fig. 31), 

 and nine plus the sex-chromosome in the other (Fig. 30). All 

 of the chromosomes, including the sex-element when it is present, 

 divide in these cells (Figs. 32-38). 



4. Spermatids. 



The division of the secondary spermatocyte gives rise in the 

 one case to spermatids containing nine chromosomes (Figs. 36- 

 38), and in the other case nine plus the sex-chromosome or ten 

 (Figs. 33 and 34). All of the chromosomes except the sex- 

 chromosome are bivalent in nature so that in reality we have the 

 equivalent of eighteen chromosomes in the one kind of spermatid 

 and eighteen plus the sex-chromosome in the other. The bivalent 

 chromosomes frequently begin to separate before the division of 

 the cell is complete. Occasionally the eighteen chromosomes 

 can be distinguished as independent elements after the cell 

 divides (Fig. 38), although the chromosomes usually disintegrate 

 before complete separation can be identified. All of the foregoing 

 evidence indicates that eighteen is the reduced number of chro- 

 mosomes. 



V. DIMORPHISM IN THE SPERMATOZOA. 



The spermatozoa of the bull vary considerably in size, and 

 careful measurements show that they may be arranged in two 

 separate classes, one type being much larger than the other. 

 Mature specimens, which were free in the lumen of the tubule 

 and parallel to the objective, were selected at random from a single 

 slide and outline sketches of six hundred heads were made with 

 the aid of a camera lucida. The lengths of the sketches were 

 then carefully measured and recorded in quarter millimeters. 

 Figure I in the text shows the variation in size of the six hundred 



