62 McCLUNG. 



but quickly condenses its substance and becomes homogeneous, 

 transparent, and sharply outlined. It early assumes a position 

 upon the periphery of the forming nuclear vesicle and maintains 

 this during all the prophase. Throughout this period, and the 

 remaining stages of its existence, it persistently stains according 

 to the reaction exhibited by chromosomes of the metaphase. It 

 is thus, in both its chemical and physical properties, strikingly 

 different from the remainder of the chromatin. It may be said, 

 therefore, that it is a chromosome of the previous generation (i. 

 c., one formed from the spireme of that generation) which exists 

 as such while its fellows pass through the prophase of another 

 mitosis. 



So far as the first spermatocytes are concerned, this is the 

 only point at which the accessory chromosome differs from the 

 others. When metakinesis occurs, all the chromatin elements 

 divide at the same time and in the same way. The participation 

 of the accessory chromosome in this act is generally conceded 

 (1 33> 36, 47), but the exact process has not yet been observed 

 because no form has been studied where the element stands 

 out conspicuously enough to be noted. From the nature of the 

 element, however, the only reasonable thing to expect would 

 be that it should divide as it has done in all previous mitoses, 

 /. i\, longitudinally. 



With regard to the action of the accessory chromosome in the 

 second spermatocyte mitosis, there are two opinions. One is 

 that it takes part with the ordinary chromosome in the act of 

 division (^[ 34), the other that it passes undivided into only one 

 of the two resulting cells (! 9, 12, 13, 37). Examination of 

 the literature shows that the weight of evidence, so far as obser- 

 vations are concerned, supports the latter view. In addition to 

 this, the fact that it differs from the other chromosomes must be 

 taken into account. When it divides in the first spermatocyte it 

 has finished the entire act of separation begun for it in the pro- 

 phase where it was formed. It has no need, therefore, to divide 

 in the second mitosis where the other chromosomes complete the 

 separation of the chromatids formed in the prophase of the first 

 spermatocyte. Thus, when the origin of the element is taken 

 into consideration, a phenomenon at first apparently inexplicable 



