BEHAVIOR OF CHROMATIN IN HYBRIDS 507 



nuclei are separate the aster is undivided (see fig. 2) and it is 

 only after complete fusion of the nuclei has taken place, as will 

 be seen in figure 3, that the asters assume a polar position like 

 that shown in figures 4 and 5. 



The stage which immediately follows shows still more plainly 

 that the pronuclei do not fuse in these hybrids. This stage is 

 shown in figure 6, and the condition represented there has been 

 found in all the hybrids of this stage. Figure 7 shows a nor- 

 mally fertilized Fundulus egg in a corresponding stage, and the 

 difference between the two is obvious. In the hybrid there are 

 two dark masses lying in the spindle, in which masses the chro- 

 mosomes can be seen. In the normal egg a single mass of the 

 same nature is present, evidently the> fusion nucleus. Taken in 

 connection with the total absence of single nucleus stages in the 

 hybrid, the evidence shows that although the germ-nuclei fuse 

 in the normally fertilized Fundulus egg, th^y do not do so when 

 the egg is fertilized with Ctenolabrus sperm. 



The chromosomes of Fundulus are much larger than those of 

 Ctenolabrus. They vary somewhat in shape according to their 

 stage of development, but they are always easily to be distin- 

 guished from the small round Ctenolabrus chromosomes. In the 

 metaphase the Fundulus chromosomes are long, but not straight. 

 They have usually a thickening at one end. This can be seen 

 in figure 10 which shows a few chromosomes from a normal meta- 

 phase group. Figure 15 shows the Fundulus chromosomes in 

 the anaphase, where they have lengthened and straightened. 

 Figure 9 shows a spindle from a normally fertilized Ctenolabrus 

 egg, with the typical round Ctenolabrus chromosomes. 



In the hybridized Fundulus egg, these two types of chromo- 

 somes are easily seen in the first cleavage spindle. In the meta- 

 phase, indeed, the chromosomes lie so close together that the 

 two types can hardly be distinguished, but as soon as they begin 

 to draw apart one sees the difference very plainly. Of course in 

 the study of these stages, sections should be selected in which 

 all the chromosomes are included. Otherwise, small* pieces of 

 the Fundulus chromosomes might be confused with the Cteno- 

 labrus chromosomes. Figure 11 shows a spindle in which the 



