54 FUNDAMENTALS OF CYTOLOGY 



because of the uncombined NH2 and COOH groups in the amino acids 

 of which they are composed. The reaction of a given cell protein 

 depends upon the relation existing between its isoelectric point and the 

 pH of the medium: it acts as an acid and stains with a basic dye if its 

 isoelectric point is below the pH of the solution. When the relation of 

 these two factors is reversed, the protein acts as a base and takes the acid 

 dye. Hence when it is desired to stain the nucleus with one dye and 

 the cytoplasm with another, success may depend upon adjustments of 

 the pH of the solutions with respect to the somewhat different isoelectric 

 points of the two regions. Exact values are difficult to determine because 

 of the chemical complexity of the protoplasmic system and the fvu'ther 

 complications introduced by other variable factors. 



One of the most useful staining techniques now used in cytology is the 

 Feulgen reaction, which consists in the restoration of color to decolorized 

 basic fuchsin by aldehyde groups in the pentose component of thy- 

 monucleic acid. It is thus rather highly specific for chromatin and can 

 be used to distinguish chromosomes from other bodies. Ribonucleic 

 acid, present in nucleoli and sometimes in cytoplasm, gives a negative 

 Feulgen test, but it can be detected through the absorption of ultraviolet 

 light which it shows in common with tl\vmonucleic acid of the chromatin. 



Conclusions.- — The matters discussed in this chapter all have a more 

 or less direct bearing on the work of the cytologist, who is aware that his 

 own understanding of every cytological object and process will be 

 deepened bj' what the physicist and chemist can help him to learn about 

 protoplasm. Complete comprehension of protoplasmic activity is a goal 

 that cannot be approached rapidly and perhaps can never be reached, yet 

 it is helpful to have in mind a provisional picture of protoplasm as a 

 physicochemical system. 



Pix)toplasm is an extraordinarily complex mixture of materials of 

 manj^ kinds, each of which has some share in determining the nature of its 

 activities. It may be thought of as a vast array of ions, molecules, and 

 molecular aggregates, some of them large enough to be visible, forming a 

 colloidal system of numerous phases. Certain proteins, because of their 

 linear molecules and chain-forming ability, seem to constitute a sort of 

 loose submicroscopic framework to which some of the lipides, phos- 

 pholipides, and other materials are attached. Lecithin, with its hj^dro- 

 phile and lipophile groups, acts as a link between proteins and fats. 

 Water molecules in great numbers, together with inorganic ions and 

 molecules, occur in the interstices of the framework. The whole mass is 

 capable of streaming because the unions between the various substances 

 in the framework are readily broken and reestablished in new waj^s. 

 Local variations of this structure occur in the membranes, plastids, 



