48 The Chromatin in the Development of Hybrids 



tainty the small chromosomes were placed in a horizontal plane toward 

 the side away from the last cleavage plane. 



9. Fourth Cleavage. — When in the fourth cleavage the chromatin has. 

 resolved itself into chromosomes the two kinds are again mingled. The 

 mingling has evidently gone farther, because in very few of the cells 

 can even a partial grouping be discovered. I have found only one cell 

 in which the two kinds of chromosomes were bilaterally distributed upon 

 the spindle. In the sectioning of this cell the knife cut in such a way as 

 to pass between the two groups so that in the one section nearly all short 

 ones were found, and in the other section nearly all long ones. In my 

 study of these sections I had well in mind the possibility that the short 

 ones might be the ends cut from the long ones of the other section. That 

 this is not the case, however, is evident from the fact that the long 

 chromosomes of the .one section have the characteristic length of the one 

 species, and those of the other section that of the other species. There 

 cannot be any doubt that we have to do here with two kinds of chromo- 

 somes, and that we can be perfectly certain these represent the distinct 

 maternal and paternal groups. The usual condition is for the chromo- 

 somes to be well mingled on the spindle. In such an anaphase. Fig. 20 

 (Plate III), the two kinds of chromosomes can be clearly made out. In 

 endeavoring to recover all of the chromosomes of each kind, I have found 

 it convenient to draw each chromosome as I followed it, retaining its re- 

 lation to some other one or more chromosomes but not its position in the 

 spindle. Figure 21 (Plate III) represents such a drawing of an ana- 

 phase of the fourth cleavage, Although, as stated above, I have been 

 unable to recover all the chromosomes, the drawing which I made as faith- 

 fully as I could with only the partial aid of a camera, serves well to show 

 the presence of two kinds of chromosomes and their mingled condition. 



10. Later Cleavage. — I have followed the behavior of the maternal 

 and paternal chromosomes from the fourth cleavage through successive 

 stages to late cleavage. Here, with often several hundred cells in any 

 given section, in all stages of division and cut in many different planes, 

 the conditions for such study are favorable. I have carefully examined 

 many thousand cells in both hybrids with the view of finding one in 

 which the two kinds of chromosomes had remained grouped but I have 

 not been able to find a single undoubted instance. On the other hand, 

 nuclei showing the two kinds of chromosomes mingled together upon 

 the s])indle are everywhere to be found. The two kinds of chromosomes, 

 naturally, cannot be distinguished in the metaphase, not even when the 

 chromosomes have begun to split. In the stage represented in Figure 32 

 (Plate IV) (lower cell to left), some of the long chromosomes may be seen 



