52 FUNDAMENTALS OF CYTOLOGY 



proteins concerned are relatively simple ones such as protamines (in fish 

 sperm) or, more commonly in animals, the somewhat more complex and 

 less basic histones. In plants the nuclear proteins are less well known, 

 but they appear to resemble histones in composition. Of great signifi- 

 cance is the previously cited fact that each type of organism studied seems 

 to have its own peculiar kind of protein. So far as nuclear materials are 

 concerned, differences between species appear to reside largely in the 

 protein portion of the nucleoprotein molecule and probably to a lesser 

 degree in the nucleic acid portion responsible for the chromatin's high 

 stainability with basic dyes. Nucleic acid is composed of chemical 

 groups of three main types: (1) phosphoric acid groups, (2) pentose 

 carbohydrate groups (sometimes hexoses?), and (3) purine and pyrimi- 

 dine bases. Nucleic acid, like the proteins with which it is associated, has 

 the remarkable ability to form long chains. Of extraordinary^ significance 

 is the recent discovery that the tobacco mosaic virus, which like other 

 viruses has the power of increasing its substance when in a protoplasmic 

 medium, is a nucleoprotein. 



The karyolymph consists mainly of proteins less highly polymerized 

 than those of the chromosomes. The nucleolus has at least two main 

 constituents: (1) a protein that does not stain with iron-hematoxylin and 

 (2) a stainable sulphuric ester of a polysaccharide. At certain stages 

 a form of nucleic acid can be detected in the nucleolus and in some cases 

 in the cytoplasm. The small amount of mineral matter in nuclei lies 

 in the chromatic elements rather than in the karyolymph, to judge from 

 the location of ash in incinerated tissues. The enormous chemical com- 

 plexity of the nucleus is evident when one considers that in a sperm cell 

 of ordinary size the nuclear portion has a dry weight of scarcely a billionth 

 of a milligram; yet this minute mass of material, which constitutes about 

 3 per cent of the weight of the living sperm head, carries the physical 

 basis of the paternal hereditarj^ contribution to the next generation. 



The Staining of Protoplasm. — The staining reactions of protoplasm 

 depend upon its chemical composition, its colloidal state, and certain 

 characteristics of the dye solutions. It is mainly the proteins that take 

 up the stains, but certain products of other kinds, such as minute fat 

 droplets, may be so abundant and stainable as to obscure the effects of 

 the stains on the protoplasm itself. The dyes employed, aside from 

 valuable natural ones like hematoxylin and carmine, are for the most part 

 coal-tar products. These dyes are commonly employed as salts and 

 fall into two main classes: basic dye solutions carry the color in the 

 cations, whereas acid dye solutions carry it in the anions. Familiar 

 examples of the former group are safranine (red), crystal violet (blue or 

 violet), and methyl green; members of the latter group are eosin (red), 

 methyl blue, and fast green. 



