SPERMATOGENESIS IN THE ALBINO RAT 149 



In lateral views of the spindle the chromosomes cannot be 

 enumerated if the sections are cut thick enough to include them 

 all, since they lie close together, and some are within the outer 

 layer, as is seen in figures 18 to 20. Furthermore, many are 

 about equal in size and shape, so that there is no means of identi- 

 fying particular ones except in a few cases, as already indicated. 

 Still another difficulty is presented by the nucleolus. The fate 

 of this body is doubtful. The evidence in the rat so far gathered 

 points to a gradual absorption, as discussed under the topic ' The 

 nucleolus,' p. 151. The faintly staining body in figure 18 may, 

 with heavy staining, look exactly like a chromosome. 



The only place where one may /be perfectly confident of counts 

 is in late prophase, just before the chromosomes assemble on the 

 spindle. They are then scattered and distinct. It is upon counts 

 made at this stage that I rely chiefly for my decision that 19 is 

 the haploid number of the chromosomes. Thirty-one entire 

 nuclei in which the chromosomes could be enumerated gave the 

 following results: 



24 cells, unquestionably 19 chromosomes and a nucleolus 

 5 cells, unquestionably 19 chromosomes, no nucleolus 

 1 cell, unquestionably 20 chromosomes and a nucleolus 

 1 cell, unquestionably 18 chromosomes, no nucleolus 



It is to be seen that these thirty-one cells give an average of 19 

 chromosomes. Added to this evidence are five equatorial plate 

 counts, one of which is from a smear preparation, each giving 19 

 chromosomes. Three of these are shown in figures 18 to 20, all 

 from different animals. As still further evidence, one cell in 

 anaphase showed 19 chromosomes at one, and 18 at the other 

 pole. 



Under the topic dealing with the shapes of the chromosomes it 

 was noted that 16 rings had been found in one cell, all but one 

 being shown in figure 27. Thirteen of these are to be seen in 

 metaphase in figure 18, with a possible 14th — C, — if that be a 

 ring. Eleven rings are clear in the cell from which figure 19 

 was made. The metaphase plates have not revealed more than 

 14 rings. It would therefore appear that we may assume 16 

 rings as probably the highest number of rings usually occurring. 



