ON THE ZOOLOGICAL STATION AT NAPLES. 331) 



as well as with what I had already found to be the case in the spermato- 

 genesis of Mammalia, and what Brauer regards as the more correct ren- 

 dering of the same phenomena in Ascaris. 



The last two divisions of the Elasmobranch spermatogenesis are quite 

 distinct from those which go before. 



TlT»re are twelve chromosomes in the penultimate division, whether 

 we count them when emerging as chromatic rings from the reticulum of 

 the previous resting nucleus in the ' monaster,' or as the divided loops 

 when nearing the opposite poles of the spindle figure as suggested by 

 Boveri. 



The daughter nuclei produced pass into a condition of complete 

 repose prior to tlie last division of the spermatogenetic series, and the 

 initial phases of the last mitosis, as well as its whole course, proceed in 

 exactly the same way as in the penultimate division, except that only six 

 chromosomes emerge from the resting reticulum instead of twelve. 



A close examination of these twelve chromosomes of the penultimate 

 division reveals the fact that they are each built up of four small conden- 

 sations or primary elements united together in the form of a ring, while 

 those constituting the individual loops or rings of the final division are 

 far more numerous. The small size and crowding of the parts in the 

 latter phase render counting very difficult, but numbers of readings 

 satisfied me that in this division they are eight — i.e., the twelve fourfold 

 chromosomes of the penultimate division are rearranged in the form of 

 six eight-fold chromosomes in the last division, while both divisions are 

 normal mitoses, in which there is no passage of unsplit chromatic masses 

 to the poles. 



In the spermatogenesis of this fish, then, there is reduction only in the 

 number of the chromosomes when they reappear from this rest in the final 

 as compared with the penultimate division. But this change is not 

 brought about by either division ; it occurs during the resting condition of 

 the nucleus between the two, and exists simply as a rearrangement of the 

 parts. 



From Brauer's work it seems that the so-called ' reduction ' in the 

 spermatogenesis of Ascaris is brought about during an intervening rest 

 between the last mitoses, just as in the case of the Mammalia, and we now 

 have it also in the cartilaginous fish, so that it follows that in these 

 widely separated groups the last two divisions of the spermatogenesis do 

 not correspond to the successive extrusions of the polar bodies, because 

 between the two mitoses which bring about this extrusion there is no rest 

 phase, and the supposed absence of this between the last divisions of the 

 spermatogenesis was the very fact from which Hertwig argued their 

 similarity. 



Moreover, Boveri seems conclusively to show that there is no ' reduc- 

 tion ' accomplished in any division up to the formation of the first ovicyte, 

 nor yet in either division by which the polar bodies themselves are formed. 

 Yet there are only half as many chromosomes in the divisions which form 

 the polar bodies as there are in the mitoses prior to the formation of the 

 first ovicyte, and so ab infra the reduction must occur in the rest after the 

 division which forms the cell. 



If a comparison can be made at all, the spermatogenesis of the forms 

 under discussion stops short at a point corresponding to the formation of 

 the first ovicyte in the ovigenesis. 



z3' 



