William J. Moenkhaus 49 



characteristically extending their ends toward the poles. The short chro- 

 mosomes still confined to the equatorial plane cannot be identified as such. 

 In nearly every anaphase, however, the short chromosomes can be clearly 

 distinguished among the long ones [Figs. 23 (Plate III) and 32 (Plate 

 IV)]. 



The two kinds of chromosomes can be distinguished not only by their 

 size but also physiologically. In Figures 22 and 32, it can be seen that 

 the short chromosomes, as a whole, are nearer to the pole than the long 

 ones. This shows most clearly in the further half of the spindle where 

 the short chromosomes remain more abreast and are, as a whole, nearer 

 the pole, forming a band across the spindle. Here, as in the earlier 

 stages of the Menidia hybrid, the long chromosomes are the stragglers, 

 being more irregular than and, as a whole, behind the short ones in their 

 migration to the poles. 



While this difference in the rate of migration comes out most strik- 

 ingly in the spindles where the chromosomes have not yet become min- 

 gled [Figs. 29, 30, 31 (Plate IV)], it is just as truly present in the later 

 cleavage cells where this mingling has taken place. The small chromo- 

 somes, in the more extreme instances, may in the telophase become com- 

 pletely separated from the long ones, as shown in Fig. 23 (Plate III). 

 This figure represents the early telophase of the third cleavage. The group 

 of small vesicles nearer the pole are doubtless the small chromosomes al- 

 ready well along in their transformation while the larger group would then 

 represent the larger chromosomes not yet so far along in their transforma- 

 tion but that some of the longer chromosomes can be identified. It will 

 occur to every one that notwithstanding the fact that the chromosomes 

 may be thoroughly mingled during the active phases of the cell cycle, 

 the two kinds may in this way become separated in the resting nucleus. 

 The reasons for believing that this does not usually occur will appear 

 below in connection with another matter. 



I have considered it important to carefully compare the conditions in 

 the hybrids with corresponding stages in the normal eggs of the two 

 parent species. Although practically all of the points above brought 

 out Avould be sufficiently evident taken by themselves they become doubly 

 so through such comparison. The question to arise is whether the dif- 

 ferences in the size of the chromosomes might not also be found in the 

 normals. This is clearly not the case. In any given cell during a phase 

 when the chromosomes come distinctly to view all the chromosomes are 

 of practically the same size. Any variations in their size cannot be 

 confounded with the differences obtaining in the hybrid egg?: Further- 

 more, the chromosomes, apart from their size, show a certain individual- 

 4 



