THE ACCESSORY CHROMOSOME. 6 1 



spermatologists and practically adopts Paulmier's views concern- 

 ing the accessory chromosome. We find in his summary that 

 " A comparison of the foregoing results indicates that the small 

 tetrad (dyad) corresponds to the extra chromosome observed by 

 Henking in Pyrrhocoris, and perhaps also to the ' accessory chro- 

 mosome ' of Xiphidium. Whether it corresponds to the ' chro- 

 matin nucleolus ' of Pentatoma is not yet clear. The most re- 

 markable of these strange phenomena is the formation of the 

 small tetrad, which seems to be a non-essential element, since it 

 does not contribute to all the spermatozoa. Paulmier is inclined 

 to ascribe to it a vestigial significance, regarding it as a degen- 

 erating chromosomes which has lost its functional value, though 

 still undergoing in some measure its original morphological trans- 

 formation ; in this connection it should be pointed out that the 

 spermatocyte nucleolus, from which it seems to be derived, is rep- 

 resented in the spermatogonia by two such nucleoli, just as the 

 single small tetrad is represented by two small chromosomes in 

 the spermatogonia mitoses. The real meaning of the phenomena 

 is, however, wholly conjectural." 



Because of the greater prominence of the element in the sper- 

 matocytes, observations upon its occurrence and changes have 

 been more numerous and accurate than in the case of the sper- 

 matogonia. During the prophase of the first spermatocyte, 

 particularly, the appearance of the element is so striking as to 

 render its oversight impossible. Regarding the main features 

 distinguishing it, there is a convincing agreement in all the pub- 

 lished reports and these speak further for the morphological ex- 

 clusiveness so plainly manifested by the element in the sper- 

 matogonia. There are some slight discrepancies in the accounts 

 of its very early appearance but concerning the later stages there 

 appears to be no confusion. 



From the Hemiptera (1 27) and the Orthoptera (1 47), we 

 learn that the chromosomes of the last secondary spermato- 

 gonia, with the exception of one, break down rapidly into their 

 constituent chromomeres and that these enter at once into the 

 formation of a spireme at first thin and fine but later coarse and 

 granular. The exception to this process is the accessory chro- 

 mosome. It is, in the beginning, somewhat irregular in outline 



