THE ACCESSORY CHROMOSOME. 67 



spermatogonial division but agrees with Paulmier's view concern- 

 ing their later changes. 



Sutton, on the contrary, has devoted his entire attention to the 

 spermatogonial divisions and has given us a fairly complete ac- 

 count of the changes taking place in Bracliystola (149-55). 



It is probable that this object presents an extreme view of the 

 accessory chromosome. From an inspection of Button's prep- 

 arations and a comparison with other forms, however, I am 

 strongly inclined to the behalf that it is merely a very marked 

 example of a normal process. We may, therefore, take the 

 course followed by the accessory chromosome in the spermato- 

 gonia of Bracliystola as representative since it is all that we have, 

 at present, to base our knowledge upon. 



Even with this we have no hint, as to the real origin of this 

 problematical body a point of great importance. It is hoped 

 that more extensive collections of material will render it pos- 

 sible to learn something of this, but the problem is one of con- 

 siderable difficulty owing to the compact nature of the early cells 

 and their apparent irregularity in arrangement. A favorable ob- 

 ject may be found, however, in which the structure will stand 

 out clearly enough to be accurately studied, and in this event we 

 may discover the conditions determining the setting apart of this 

 one chromosome from all the others. 



Sutton was first able to distinguish the accessory chromosome in 

 cysts of eight or sixteen cells. Here, as in the spermatocytes, it 

 seems to be removed as far as possible from the influence of the 

 ordinary chromatin. This is accomplished by an enclosure in a 

 separate vesicle which, as Sutton observes, may almost be re- 

 garded as a separate nucleus (^[ 51). A distinct existence is 

 maintained during all the stages when a possible exchange of 

 material between the accessory chromosome and the other chro- 

 mosomes might be accomplished. Only after the chromosomes 

 are definitely established as independent bodies are the barriers 

 removed and then only long enough to permit the act of meta- 

 kinesis to take place. 



During the period intervening between the acts of division, the 

 conduct of the accessory chromosome parallels that of the 

 nucleus containing the remainder of the chromatin (^ 50). A 



