THE ACCESSORY CHROMOSOME. 69 



are many facts which argue against it. Of these I should like 

 to speak in some detail, since the theory they controvert is the 

 only one yet advanced upon the proper basis that the element is 

 a chromosome and not a mere nucleolus. 



Paulmier considers the element a chromosome in the process 

 of disappearing from the species and reaches this conclusion 

 after observing that in the last spermatocyte mitosis it fails to 

 divide and is thus unequally apportioned to the resulting sperma- 

 tozoa. Before considering a theory based upon so unusual a 

 phenomenon as this, it would be well to make certain that it is 

 an actual and common occurrence. That it is must be granted, I 

 think, after the work of Henking, Paulmier, and myself, upon so 

 many different forms, has shown it to be of such wide distribution. 

 Granting this, then, there remains to be examined the validity of 

 the assumption that the act presages the final extinction of an 

 element. 



In combating the suggestion of Paulmier, I shall make use of 

 the evidence offered in the different cell generations, commencing 

 with the spermatogonia. Every instance, according to my inter- 

 pretation, shows facts strongly incompatible with this author's 

 view. I would suggest in this connection that the extreme im- 

 portance of the structure is unmistakably manifested by its course 

 in the spermatogonial divisions. What could more strongly 

 emphasize the special importance of an element than to have it 

 set apart in a separate vesicle while its fellows are provided with 

 one common investment ? Morphologically, this is simply rais- 

 ing the accessory chromosome to the rank of a nucleus co- 

 ordinate with the one commonly present in a cell. Its careful 

 and uniform division during the mitoses of all the spermatogonia 

 suggests anything but an unimportant structure. Had we no 

 further refutation of the degeneration theory than that afforded 

 by the spermatogonia, it would, I think, be sufficient. The 

 evidence of the other cell generations, however, strengthens this 

 position and is well deserving of attention. 



From the spermatogonia, each spermatocyte receives one 

 chromosome (the accessory) which through all the subsequent 

 stages exists as a chromosome and never suffers extensive disturb- 

 ance of its chromomeres either for the purpose of metabolic activi- 



