1 88 THE POPULAR SCIENCE MONTHLY 



tion; up to a certain stage, the nuclei of such cells shrink in size and 

 become more densely chromatic in proportion as the cell protoplasm 

 is converted into metaplasm. The same thing is true of gland cells, 

 muscle cells, fiber cells and fat cells in which the general protoplasm 

 is progressively being changed into differentiation products, and coin- 

 cidently the individual nuclei shrink in size and become more densely 

 chromatic. 



In no case do metaplasmic substances or differentiated structures of 

 the cell enter into the nucleus during its growth, and the relative quan- 

 tities of general protoplasm and of differentiated products in a cell can 

 be determined by the size to which the nucleus will grow during inter- 

 kinesis, under given conditions of time, temperature, etc. By sub- 

 jecting eggs to centrifugal force, the quantities of protoplasm and yolk 

 in the cleavage cells may be greatly changed, and under such circum- 

 stances the size of a nucleus is always proportional to the volume of the 

 protoplasm in which it lies; the heavier yolk which segregates at the 

 peripheral pole, and the lighter watery or oily substance which gathers 

 at the central pole of the centrifuged egg do not contribute to nuclear 

 growth, only the clear protoplasm which lies in the middle zone enters 

 the nucleus or contributes to its growth. In muscle cells with small 

 nuclei, the quantity of general protoplasm (sarcoplasm) which may 

 enter into the nucleus or contribute to its growth is small; in nerve 

 cells, it is evidently larger, since the nuclei of such cells are relatively 

 large, but the substance which may enter the nucleus of a nerve cell is 

 by no means as great in quantity as in germ cells and blastomeres, thus 

 indicating that much of the substance of a nerve cell is too highly differ- 

 entiated to enter into the nucleus. In epithelial and gland cells, the 

 size of nuclei is limited not only by the presence of metabolic products 

 in the cells, but also by the occurrence of cell division and the conse- 

 quent limitation of the growing period of the nucleus. 



The following table gives the cell diameter and nuclear diameter at 

 maximum size, the corresponding nuclear volume, the cell volume less 

 the nuclear volume, and the nucleus-cell ratio, in a number of different 

 kinds of cells in adult individuals of Crepidula plana: 



The nucleus-cell ratio of these cells varies from 1 : 1.3 to 1 : 88.6, 

 depending primarily upon the quantity of formed substance in the cells. 

 The nuclei are relatively largest in germ cells before the formation of 

 yolk, and in embryonic cells in which there is relatively little formed 

 substance; in such cases a relatively great part of the protoplasm may 

 enter the nucleus. The nuclei are relatively smallest in those cells in 

 which the protoplasm has been most completely transformed into pro- 

 ducts of metabolism or differentiation, such as gland cells filled with 

 secretion, red blood cells of mammals in which the nuclei completely 

 disappear, egg cells filled with yolk, and spermatozoa in which most of 



