92 bulletin: museum of comparative zoology. 



divisions. Montgomery in his later papers adopts the conclusions of 

 1'aulmier, believing with him that it is a chromosome undergoing the 

 process of elimination. McClung (:02 a ), in a paper dealing in detail with 

 all of the reported observations upon the accessory chromosome, formu- 

 lates an hypothesis which ascribes a very different function to this ele- 

 ment. He maintains that the mere fact that the chromosome enters 

 into the formation of only half of the spermatozoa would not necessarily 

 indicate that the element is degenerating, and, in addition, that there are 

 other facts which militate strongly against such a conclusion. The ex- 

 treme nicety with which this element is excluded from contact with the 

 others at all stages, and especially in the spermatogonia, would seem to in- 

 dicate a very different and a much more important significance. This 

 exclusiveuess taken in connection with the fact that exactly one half the 

 spermatozoa contain this element, suggests the theory that it has to do 

 with the determination of sex, as this is the only respect in which the 

 progeny are divided into two classes of equal number. Although no 

 positive proof is advanced to support this theory, the author establishes, 

 in a very logical manner, the probability of the accessory chromosome 

 exercising such a function. It seems to possess all of the character- 

 istics required of such an element. Definite proof of the function of 

 this structure can be obtained, however, only by a study of the processes 

 occurring in the fertilization of the egg. 



My observations upon the accessory chromosome in Scolopendra have 

 added very little to our knowledge of this element, except in so far as 

 they demonstrate its wide distribution and the great similarity of its 

 behavior in widely separated groups. Indeed, in all important partic- 

 ulars, the phenomena accompanying the development of this structure 

 are identical in Chilopoda and Orthroptera, although in minor details 

 the processes vary considerably. In both groups the element is derived 

 directly from a single spermatogonial chromosome, and for this reason 

 takes no part in the phenomena of synapsis. During the prophase, 

 when the other chromosomes divide into four chromatids and form 

 tetrads, this element, as would be expected from its origin, cleaves only 

 once and then longitudinally. In the two succeeding divisions it is 

 divided only once, and thus is present in but one half of the spermatids. 

 The differences, although at times puzzling, are in reality slight and 

 unimportant. Thus, at the time when all of the chromatin is aggregated 

 in the karyosphere, the accessory chromosome cannot be distinguished, 

 except in cases of the most favorable sections ; but from the study of 

 these thin, well-differentiated sections one is justified in saying that 



