82 McCLUNG. 



elements, a tendency to depart from typical appearances. Mont- 

 gomery has investigated the spermatogenesis of Pcripatus and de- 

 clares that the "chromatin nucleolus" is not present in that form. 

 His evidence, however, I regard as not convincing in this respect. 



Credit is assigned Montgomery (^ 24) for the discovery that 

 the accessory chromosome is merely a spermatogonial chromo- 

 some that comes over unchanged into the spermatocytes. This 

 must now be withdrawn, for, at present, he accepts the views of 

 Paulmier that it is formed by synapsis from differentiated elements 

 of the spermatogonia. That it is, on the contrary, the same in 

 both cell generations, at least in the Orthoptera, is shown by the 

 work of Sutton (If 1 49, 52). 



Perhaps the most important advance in our knowledge of the 

 accessory chromosome pertains to its relation to the other chro- 

 mosomes of the cell. During the early investigations upon it, 

 the tendency was to consider it widely removed from the type 

 chromosome, but we are now beginning to perceive that practi- 

 cally its only divergence consists in its isolation. This feature is 

 most pronounced in the prophase of the first spermatocyte where 

 the element bears some little resemblance to a nucleolus enough, 

 in fact, to have induced several investigators to so call it. But, 

 as has already been shown, this is merely superficial, and later 

 researches upon the Locustid cells have brought to light the fact 

 that the accessory chromosome forms a close spireme of its own, 

 and so parallels the activities of the ordinary chromosomes at the 

 point where it seemed most to diverge. We may therefore re- 

 gard the accessory chromosome as practically normal in its be- 

 havior throughout the different cell generations of the testis up 

 to the point where it is thrown into prominence by the unusual 

 action of the remaining chromosomes during the pseudo-reduc- 

 tion. It may well be, as previously suggested, that the distinc- 

 tion bestowed upon the accessory chromosome at this time is due 

 to its fidelity to the type form of division which, at this point, is 

 abandoned by its fellows. 



Regarding the theory of its function advanced in this paper, I 

 can say only that it has, if anything, been strengthened by later 

 researches, and more nearly explains the phenomena involved 

 than any other that has been conceived. 



