August 18, 1916] 



SCIENCE 



221 



cleated cells can synthetize fats, carbohy- 

 drates, and proteins in general, including 

 the most complicated compound forms. 

 What real evidence have we that nucleo- 

 proteins constitute the sole exception? 4 



If we reckon with the synthetic powers 

 of the cytoplasm as a possibility, we must 

 next inquire how these can be influenced 

 by the presence of a specific nucleus. That 

 cytoplasmic response, in general, is de- 

 pendent on the chemism of the cell, and 

 that these activities are specifically and 

 profoundly modified by changes in the va- 

 riety of nuclear material present, are well- 

 known facts shown nowhere more clearly 

 than in the structural differentiations 

 called forth in hybrids. These, especially, 

 are important for us since the introduction 

 of nuclei into foreign cytoplasm demon- 

 strates most strikingly their ability to regu- 

 late syntheses so that more nuclei like 

 themselves are produced. In what terms 

 are we to conceive this regulation ? 



The influence of a specific chromatin on 

 cellular processes can be directly attrib- 

 uted to the samples which are known to 

 leave the nucleus and come directly into 

 the cytoplasmic reaction-sphere. But the 

 details of their activity there remain ob- 

 scure. Autocatalysis, suggested on quite 

 inadequate grounds, is not necessarily ex- 

 cluded by the recent work of Conklin 5 and 

 other effects are also thinkable. A fitness, 

 chemical or physical in nature, between the 

 liberated chromatin or its products, on the 

 one hand, and certain of the reaction-prod- 

 ucts of cytoplasmic synthesis on the other, 

 leading to the formation of different, or 



4 For a fuller discussion of the methods, evidence, 

 and conclusions, see the articles by A. B. Macallum 

 in Abderhalden, "Handb. d. Bioehem. Arbeit- 

 methoden, " and in Aseher-Spiro "Ergebnisse d. 

 Physiol.," VII. 



5 E. G. Conklin, Journal of Experimental Zool- 

 ogy, Vol. 12, pp. 1-98. 



larger, non-reacting aggregates, would 

 automatically increase the production of 

 such substances, provided always the ma- 

 chinery necessary for their production is 

 given at all. Very possibly the reciprocal 

 relation suggested here is one of the keys 

 to successful hybridization. 



It is useless to hope for intellectual satis- 

 faction in this matter at the present time. 

 We can, however, assert with confidence 

 that a cell is viable and assured of the pos- 

 sibility of offspring, essentially like itself, 

 if it contains, at the beginning of its life- 

 cycle, samples of all the various kinds of 

 chromatin possessed by its immediate par- 

 ent, and moreover, contains these in quanti- 

 ties sufficient to influence cytoplasmic syn- 

 theses so that they shall ultimately yield a 

 chromosomal complex in which the original 

 proportions among the several variants are 

 quantitatively preserved. 



THE SYNTHESIS OP CHROMOSOMES 



If chromatin or its immediate forerun- 

 ners are cytoplasmic in origin, how do they 

 get into the nucleus? The impermeability 

 of nuclear membranes for most constitu- 

 ents of the cell is probable; likewise, their 

 permeability for nucleins, since these, even 

 in the form of visible aggregates, seem to 

 pass freely into the cytoplasm. If they 

 can get through the membrane, going out, 

 they can also get through, going in. The 

 nucleus, therefore, is to be thought of as a 

 kind of sanctuary into which certain pro- 

 teins may enter, and, so long as they re- 

 main behind their wall, be free from the in- 

 fluence of other substances (Macallum). 



These considerations are only an enter- 

 ing wedge. We infer a specific chromatin 

 for each race, for every individual, and 

 even for particular cells of the individual. 

 More than this, in its intranuclear state, 

 the chromatin is organized, in all likeli- 

 hood, permanently, into chromosomes which 



