SPERMATOGENESIS IN THE ALBINO RAT 161 



are seldom seen in lateral views. The small and medium-sized 

 chromosomes usually group themselves inside the circle of the 

 large ones. 



In passing, it is of interest to note that generally the chromo- 

 somes are very closely aggregated, but occasionally one finds a 

 tubule in which they are much more widely spaced. These 

 latter are of course particularly valuable for study. The differ- 

 ence does not seem explicable on the ground of inequality of 

 fixation, as two tubules lying side by side at the edge of a block 

 of tissue will show these opposite conditions. 



6. The nucleolus. Two bodies other than chromosomes are 

 found staining like chromatid. These are the nucleolus, or 

 plasmosome, and the chromatoid body, the latter found in the 

 cytoplasm. The nucleolus is early differentiated, being first 

 identified in the pachytene stage (fig. 1.) It stains heavily 

 throughout its history up to late metaphase. By this reaction 

 and its spherical form it can be traced to a late period in the first 

 spermatocyte history. About the period when the chromosomes 

 have formed in the equatorial plate, or just after, it disappears. 

 Its definite form as last seen is shown in figure 27, when the form 

 seems to be spherical with the stainable substance densely aggre- 

 gated about the periphery. Its size is then less than in its early 

 appearance. I have seen nothing to indicate that it fuses with a 

 chromosome. It does not reappear in the second spermatocyte 

 cells. 



The cell from which figure 18 was made shows an irregular 

 body among the chromosomes (Nu.), its diffuse character indi- 

 cated in the drawing by dots. It is stained very faintly by 

 haematoxylin. This body appears in other cells at this stage with 

 the same staining reaction, but is sometimes elongated in form. 

 It may be the nucleolus, which is being absorbed. Its staining 

 reaction does not permit it to be interpreted as a chromosome. 

 This body needs further investigation. 



7. History of the accessory. Reference has been made to this 

 chromosome from time to time. Its history is readily outlined. 

 At the time of its earliest positive identification the other chromo- 

 somes are in the leptotene stage (fig. 1). It is distinguished 



