52 The Chromatin in the Development of Hybrids 



form so nearly related to Eiickert's and that of Zoja's and mine, it may 

 well be questioned whether the double and bilobed nuclei of Riickert 

 really are any indication of the distinctness of the maternal and paternal 

 chromatin. These conditions should at least make us cautious against 

 accepting too readily the conclusions based on the mere presence of 

 double and bilobed nuclei, double nucleoli and the like without any 

 further means of identifying them with the maternal and paternal 

 chromatin. Further work on a large number of forms is desirable to 

 see whether it is the rule for the parental chromosomes to become mingled 

 in these early cleavage stages. 



12. Maternal and Paternal Nucleoli.- — The recent studies of Hacker, 

 02, on parental nucleoli in different Copepod crustacean forms, has 

 already been mentioned. The rather constant presence of two nucleoli 

 in the nucleus he takes as an index of the separateness of the maternal 

 and paternal chromatin. According to this idea, one of the nucleoli 

 would represent the chromatin of the one parent and the other that of 

 the other. At the time that his preliminary paper appeared I had been 

 working on the nucleoli in fish hybrids. Most of the nuclei in the rest- 

 ing stage, when not too young, show two nucleoli. In the reconstruction 

 of the nucleus the smaller chromosomal vesicles at first fuse into larger 

 ones. In this stage one can often see a number of nucleoli in each 

 nucleus. This multinucleolate condition is followed by a binucleolate 

 condition as the fusion of these larger vesicles is finally completed. Each 

 vesicle seems to forai a nucleolus so that the number of nucleoli present 

 in a nucleus is in a general way an index of the number of vesicles com- 

 posing the nucleus. Observations of this kind, it seems to me, strengthen 

 Hacker's position. Two nucleoli would indicate that the nucleus is 

 essentially composed of two vesicles or units of some kind, although 

 these could not be distinguished in any other way. I have endeavored 

 to make out some constant difference in the size or structure of these 

 nucleoli in the hybrids but without success. In cells of the same section 

 all conditions obtain in the size, from a strongly unequal to perfectly 

 equal nucleoli within the same nucleus. This interpretation of the 

 nucleoli by Hacker has much in its favor. In such forms as he studied, 

 in which he maintains that the two parental chromosomes remain bilat- 

 erally distributed, it is easier to conceive how the nucleoli might repre- 

 sent the two paternal chromatins. In the fish hybrids under consid- 

 eration, however, where the binucleolate condition is probably just as 

 constant for the cells as in Cyclops, but where I have shown that the 

 two chromatins do not remain bilaterally distributed but both kinds are 

 scattered through the nucleus, it is difficult to believe that the scattered 

 chromosomes of a given parent are represented by a common nucleolus. 



