196 KANSAS UNIVERSITY SCIENCE BULLETIN. 



It is very much larger than most of the other chromosomes, as 

 may be seen in figure 28. It divides readily in metakinesis, 

 and its chromatids travel to the poles with those of the other 

 chromosomes, but, on account of their greater length, project 

 downward from the mass (fig. 31). Here, as always, the ac- 

 cessory stubbornly maintains its independence, and can be seen 

 extending out from the mass of other chromosomes at each end 

 of the mother cell (fig. 32). 



The division of the other class of second spermatocytes is, 

 of course, unaccompanied by modifications due to the presence 

 of the accessory chromosome. Aside from this, no difference 

 between cells of the two classes is noticeable. 



To summarize, we may say, that resulting from the division 

 of each first spermatocyte are two second spermatocytes, one of 

 which contains an accessory chromosome while the other does 

 not. The second spermatocyte containing the accessory di- 

 vides, and with it the accessory, so that each of the spermatids 

 derived from it contains a chromatid from the accessory. The 

 other second spermatocyte, not containing the accessory, also 

 divides, producing two spermatids in which the accessory is 

 absent. Thus half of the spermatids contain accessory chro- 

 mosomes while the other half does not. 



(e) Number of Chromosomes. 



The enumeration of the chromatic elements, while a very im- 

 portant part of any study upon the nucleus, is unsatisfactory at 

 the best. If there is any great number of chromosomes in the 

 cell, it is impossible to secure a determination of it in a lateral 

 view of the metaphase, because the elements overlie one another 

 so as to render their distinction very uncertain. A polar view 

 is much more desirable, but even here one is never certain that 

 all the elements are represented, or that only entire chromo- 

 somes of one cell are present. The first of these contingencies 

 arises from the fact that, in the event of a cell being cut in two, 

 some of the chromosomes may drop out and not appear in the 

 sections ; or, if still on the slide, and in a small group, they 

 may lie so close to a mass of chromosomes in another cell as to 

 be confused with them. An excess in number may be found if 

 a portion of the chromosomes have already divided in the equa- 

 torial plate, while the remainder are still united (c/. fig. 19), 



