MAMMALIAN SPERMATOGENESIS — OPOSSUM 31 



to be passing undivided to one pole. That these bodies are false 

 accessory chromosomes is proved by the fact, first, that the X-Y 

 elements (the true sex-chromosomes) can be clearly seen in most 

 cells, and, secondly, that these false accessory chromosomes 

 usually have the typical shapes of tetrads which reveal their 

 true nature. If the preservation of the material were not so 

 good, these tetrads would appear as more or less bipartite, and 

 would doubtless be interpreted as a true accessory chromosome 

 if the X-Y elements were not in prominence. 



The cause of this occasional displacement of tetrads, which 

 one observes here and there in the spindles, is probably due to 

 the technique employed (either the sectioning razor or to diffu- 

 sion currents set up during fixation and subsequent treatment of 

 material) and doubtless does not occur in the living cell. 



2. Sex-determination in mammals 



The opossum has long been cited as a mammal in which we 

 have the typical X-0 type of sex-chromosome for the male. And 

 since the appearance of Jordan's work in 1911 many investigators, 

 reporting on the spermatogenesis of other mammals, have de- 

 scribed a body similar in appearance and behavior to the ' acces- 

 sory chromosome' found by Jordan, and have interpreted it, 

 as Jordan did, as the true sex-chromosome. It may be added 

 that in most cases no attempt was made to w^ork out the complete 

 history of the chromosomes in the way Jordan had done. And 

 yet as the present work seems to prove clearly the true sex- 

 chromosomes of the opossum are of the X-Y type, and that 

 what Jordan saw was probably a displaced tetrad, or possibly 

 half a tetrad which had passed early to one pole, the other half 

 remaining in the equatorial plane of the spindle. Both of these 

 conditions have been found by the author in his opossum 

 material. In view of these facts, it seems that we may, with all 

 fairness, raise the question, whether or not in some of the other 

 mammals the same error of interpretation has not been made; 

 in brief, whether or not the true sex-chromosome has been found 

 for many forms. 



THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 35, NO. 1 



