PROTOPLASM AND CELL 49 



membrane. This membrane, like the plasma membrane, consists of 

 a part of the protoplasm whose density is somewhat greater than the 

 adjacent portions. The protoplasm which constitutes the nucleus is 

 usually known as karyoplasm. The more nearly fluid, transparent 

 portion of this is karyolympJi, 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 kno^vn 

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

 of the nucleus. In instances where the chromatin is fibrillar, the 

 threads of it are called chromonemata. During division of the cell 

 this granular material becomes arranged into definite bodies, the 

 chromosomes. It is generally thought that in these bodies are lo- 

 cated the units of material (genes) which function in the trans- 

 mission of hereditary characteristics from one generation to the 



^■c^;■^^l^, 



t^--; ir^:.:-fv.; /l'M 



tJi-,'*'-" i'^-f ■•.■■.'•■ •■ *^Vi« 



A 



d e £ g 



Fig. 12. — The upper surface of a fully developed salivary gland chromosome 

 (large vesicle type) from Simulium fly larva. The longitudinal, threadlike bands 

 are called chromonemata, and these consist of a linear series of granules, the 

 chromomeres, which have a specific arrangement of grouping. A is a semidiagram- 

 matic representation of the types of chromomeres and the ways in which they 

 are connected. At a in the main figure there are two rows of dotlike chromomeres 

 which tend to associate in pairs. The band labeled b is composed of 15 or 16 

 vesiculated chromomeres closely pressed together, c to h are other groupings of 

 chromomeres along the chromonemata of the chromosome. (From Painter and 

 Griffen: Chromosomes of Simulium, Genetics 22: 616. 1937.) 



next. There are usually one or two knots of more dense chromatin 

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

 most nuclei have another 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 



