158 EZRA ALLEN 



light of recent work on other forms. Therefore the number 38 

 (37 for the male) given in this paper is to be regarded as the basal 

 number about which variations may occur. That a variation is 

 to be expected also in the first spermatocyte chromosomes is 

 doubtful. 



The accessory. No worker on the rat has mentioned an 

 accessory. Workers on the mammalia who have recorded the 

 presence of one or more accessories have not analyzed the chrom- 

 osome organization. Usually the figures bear evidence of poor 

 fixation. In most cases the accessory is determined by either 

 precocity of movement or irregularity of position of one or more 

 chromosomes or by counts. Since the counts are not reliable 

 in poorly fixed tissue, and since both precocity of movement and 

 position are uncertain criteria concerning an accessory, there 

 would remain only the work upon human tissue for discussion. 

 As with the total number of chromosomes, there is also disagree- 

 ment with regard to one or two accessories. For this reason 

 discussion upon this feature may better be omitted in a paper 

 upon the white rat, especially since none of these observers have 

 attempted to trace the history of their accessory chromosome 

 or chromosomes, and I have no observations of my own to offer 

 upon human tissue. 



In this connection it is worthy of note that Von Ebner's figures 

 ('88) for the rat show the accessory and the nucleolus clearly 

 in the early stages of first spermatocyte development, and that 

 Regaud's ('01, '10) do the same. 



The striking similarity between the organization of the eu- 

 chromosomes in the rat and in the Acrididae is found also in the 

 accessory. It is rod-shaped; in the first spermatocytes it isolates 

 itself early, previous to the pachytene stage; throughout early 

 prophase it lies close to the nuclear membrane within a large 

 space; it is characteristically woolly and of lighter staining reac- 

 tion, and it does not divide in the first division. Further refine- 

 ment of technique may reveal it in the spermatogonia. 



