MAMMALIAN SPERMATOGENESIS — OPOSSUM 33 



the evidence is not convincing, and in view of the results obtained 

 for the opossum it is to be desired that the other mammals where 

 the old technique was used should be reworked with the newer 

 methods of Allen and of Hance. Only when this is done shall 

 we be able to accept as settled the question of the sex-chromosome 

 complex of any given mammal. 



3. Double reduction 



Is 'double reduction ' of the second spermatocyte chromosomes 

 a process which normally occurs in spermatogenesis, or may we 

 take the term as being synonymous with poor preservation and 

 technique? As far as the opossum goes, and the author suspects 

 this will hold for all vertebrates, it is perfectly certain that a 

 fusion of the chromosome elements is only obtained when poor 

 preservation is in evidence. In case the material is badly fixed 

 one may get not only double, but complete reduction into one 

 chromatin mass! 



The following facts are illuminating. In the case of the 

 opossum, it has been found that it was useless to attempt to 

 study any second spermatocyte spindles which did not lie near 

 the periphery of the testicular tubule. If one will observe this 

 precaution, absolutely no evidence for any fusion of the chro- 

 mosomes will be found. On the other hand, if the second sper- 

 matocyte spindles lie deep within the tubule, the chromosomes 

 are all more or less fused, or tangled up so that accurate study 

 of the elements is impossible. It seems clear that the explana- 

 tion for this is a matter of the quickness with which the cells are 

 killed. If they lie on the periphery of the tubule, then they are 

 quickly killed, and their true form retained. But if they lie 

 deeper, so that penetration goes on slowly, then a great deal of 

 distortion and fusion will take place before fixation is complete. 



It is believed, therefore, that, as a normal process, there is no 

 such phenomenon as 'double reduction,' and that when any 

 investigator obtains such a fusion of chromosome elements, he 

 had better take steps to insure more rapid and complete pene- 

 tration of his fixing fluid. 



