PROTOPLASM AND THE CELL 61 



portion of this is haryolymph, or nuclear sap, while the meshwork 

 of fine fibers extending through it is called linin net. Supported 

 on this net is a dark-staining granular or fibrillar substance known 

 as chromatin, which is thought to be the center of functional activities 

 of the nucleus. The threads of chromatin are called cliromonemata. 

 During division of the cell this granular material becomes arranged in- 

 to definite bodies, the chromosomes. It is generally thought that in 

 these bodies are located the units of material (genes) which function 

 in the transmission of hereditary characteristics from one generation 

 to the next. There are usually one or two knots of more dense chroma- 

 tin in the nucleus which are called karyosomes. Then besides these, 

 most nuclei have anotlier body composed of material thought to be 

 temporary storage products of nuclear metabolism, the nucleolus, or 

 plasmosome. Mitochondria, similar to those of the cytoplasm, are 

 also found in the nucleus. The cell is often spoken of as the unit 

 of structure and function in living material. Both nucleus and 

 cytoplasm are necessary for its normal activities. It is not entirely 

 possible to define the part each plays in the metabolism of the whole. 

 Since the development of the microdissector by Dr. Chambers, it 

 is possible to dissect the nucleus of a cell. Cells that are deprived 

 of their nuclei are unable to carry on assimilation, although catabo- 

 lism goes on until the cytoplasm is depleted. 



Cell Division 



The cell is limited in its size, as is the complete organism. This 

 limit of size is fixed primarily by the physiological necessities 

 which are transmitted through the surface of the cell. There is a 

 definite relation between volume and surface in any mass of mate- 

 rial, and this may be expressed in a ratio. With variation of the 

 size of the mass, the volume varies according to the cube of the 

 diameter while the surface area varies according to the square of the 

 diameter. When the limit of growth is reached the cell divides, 

 and this restores the proportion of the surface area to volume that 

 will again permit growth. Remak, in 1855, was the first to describe 

 cell division. His idea was that the nucleolus split first, then the 

 entire nucleus, and finally the cytoplasm divided, placing each por- 

 tion with its share of the nucleus. This direct method of division 

 was called amitosis. Its actual occurrence is quite rare. The usual 

 method of cell division is far more complex and less direct. There 

 are several preliminary changes or phases which must occur before 

 the actual cleavage of the cell into two new ones. This is mitotic 



