1 9© THE POPULAR SCIENCE MONTHLY 



muscle fiber. These facts seem to me to justify the conclusion which I 

 reached in a former paper : 14 



It is probable that the contractile substance which makes up the larger part 

 of the muscle cell does not contribute to the growth of the nucleus as does the 

 protoplasm of embryonic cells — that so far as the growth of the nucleus is con- 

 cerned it acts as does yolk, oil, membranes, fibers or other products of metabolism 

 and differentiation. If only the sarcoplasm of the muscle cell and not its con- 

 tractile substance is able to contribute to the growth of the nucleus, the small 

 volume of the nuclei as compared with the entire cell would find a ready explana- 

 tion. There can be no doubt that the plasma is the chief seat of differentiation, 

 as Minot has emphasized, and that highly differentiated cells, such as muscle, 

 nerve, and some kinds of connective tissue, have a larger amount of plasma 

 and its products, relative to the nucleus, than have embryonic cells. In the case 

 of fiber cells, fat cells and probably muscle cells, the cell body becomes filled 

 with the products of differentiation and metabolism, which like the yolk in egg 

 cells, or the secretion products in liver cells can not enter the nucleus and con- 

 sequently do not influence its size. In such tissue cells the cell body is relatively 

 much greater as compared with the nucleus, than in purely protoplasmic cells, 

 but I have been unable to find any evidence that the ratio of protoplasm (using 

 this term in its usual sense) to the nucleus is greater in tissue cells of Crepidula 

 than in the blastomeres. 



Just as the size of a nucleus in any given species is proportional to 

 the volume of the general protoplasm, so the volume of its chromosomes 

 is proportional to the volume of the nucleus. The number of chromo- 

 somes and their relative sizes are characteristic for each species, but the 

 absolute size of chromosomes depends upon the size of the nucleus from 

 which they come. Throughout the period of cleavage, as the cells and 

 nuclei grow smaller, the chromosomes also diminish in size. The view 

 of Boveri 17 that the chromosomes divide when they have grown to their 

 original size before division, and that thereby a definite specific size of 

 the chromosomes is maintained, finds no confirmation in the work of 

 Erdmann, 18 ' 19 Schleip 20 or myself ; while the view of Koehler 21 that the 

 autonomy of the chromosomes may be extended to their growth, which is 

 supposed to be independent of that of other cell constituents, is flatly 

 contradicted by the facts. 



During the cleavage stages at least, neither the nuclei as a whole nor 

 the chromosomes double in volume at each successive division as is so 

 often assumed. The total volume of the nuclei at the 70-cell stage of 

 Crepidula plana is only 2.25 times their volume at the 2-cell stage. The 



IT Boveri, Zellenstudien V., Jena, 1905. 



18 Erdmann, " Experimentelle Untersuchungen der Massenverhaltnisse, etc.," 

 Arch. Zellforsch., 2, 1908. 



10 Erdmann, ' ' Qualitative Analyse der Zellbestandteile, etc., ' ' Ergeb. Anat. 

 Entw., 20, 1912. 



20 Schleip, ' ' Das Verhalten des Chromatins, etc., ' ' Arch. Zellforsch., 7, 1911. 



21 Koehler, ' ' Ueber die Abhangigkeit der Kernplasmarelation, etc., ' ' Arch. 

 Zellforsch., 8, 1912. 



