William J. Moenkhaus 53 



13. The Persistence of the Individual Chromosome. — The question 

 whether the individual cliromosome persists through the resting stage 

 so that upon the resolution of the reticulum into the chromosome the 

 same component chromatin granules again go together to make the same 

 chromosome from which they were derived is a question first raised by 

 Eabl, 85, and later definitely stated by Boveri, 88. Since that time so 

 much evidence has accumulated going indirectly to support this conclu- 

 sion that it has come to be rather generally accepted. Even a general 

 review of this evidence is unnecessary here. Such a review would show 

 that the fact has never been definitively demonstrated. Some of th.e 

 most direct evidences yet given are the observations of Herla, 93, and 

 Zoja, 95, on the Ascaris hybrids in which it was shown that the small 

 chromosome of the variety univalens which entered the resting nucleus 

 with the larger ones of the variety bivalens again emerged in its char- 

 acteristic form. Equally stroag evidence is now afforded by my own 

 observations on hybrid fishes. Here, as in the Ascaris hybrids, two 

 kinds of chromosomes enter tJie resting nucleus from which each kind 

 again emerges. As long as the two kinds remain grouped, as during 

 the first two divisions, this fact has little added significance, since within 

 each group it would be perfectly possible for the component chromosomes 

 to exchange chromatin granules during the resting p'^riod. If, how- 

 ever, as occurs in later cleavage, the two kinds of chromosomes become 

 mingled the chromatin granules of both kinds must lie mingled together 

 within the resting nucleus. If from such a nucleus the two kinds of 

 chromosomes again emerge it amounts almost to a demonstration that 

 the chromatin substance of a given chromosome forms a unit and that 

 this unit persists. 



It should be mentioned here that the hypothesis of Boveri of the inde- 

 pendence of the parental chromosomes has not received universal sup- 

 port. Prominent among those who have held in varying form the oppo- 

 site view, namely, that the two parental chromatins become fused and 

 mixed either at the time of fertilization or during development, is 

 Hertwig. Hertwig, 87, maintained that fertilization demanded the 

 thorough mixing of the sperm chromatin with the egg chromatin. 

 Later, 90, he revised his view in that he no longer considered it essen- 

 tial that this fusion takes place at the time of fertilization but that it 

 nevertheless took place later, during the earlier stages of development. 

 Wilson and Mathews, 95, from their studies on the fertilization of 

 Echinoderm species, concluded that because the fusion of the two pro- 

 nuclei is here so thorough it would be impossible to maintain 

 that the two chromatin masses remained distinct. 



