70 OUTLINES OF EVOLUTIONARY BIOLOGY 



delicate protoplasmic threads, known as the linin network, the 

 meshes of which are filled with a clear, probably liquid ground- 

 substance known as nuclear sap. These two together seem to 

 differ but little from the cytoplasm which lies outside the 

 nucleus. 



The most characteristic constituent of the nucleus is another 

 substance, to which, on account of the readiness with which it 

 becomes coloured by certain dyes, the name chromatin 1 has been 

 given. This substance usually occurs in the form of small 

 granules (ch.g.) scattered dver the linin network, so that when 

 very close together they appear to form a chromatin network, 



eft 



FIG. 30. Diagram of a typical Cell. 



ch.g., chromatin granules; c.m., cell- membrane ; c.s., centrosomes lying in centrosphere 

 cyt., cytoplasm ; n.m., nuclear membrane; nu., nucleus; nucl., nucleolus. 



while not infrequently a specially large aggregation of chromatin 

 substance forms a nucleolus or karyosome (mid.). 



Owing to the presence of this chromatin the nucleus as a whole, 

 under low powers of the microscope, appears to be deeply coloured 

 by such stains as various preparations of logwood and carmine 

 and the basic aniline dyes. We have already had occasion to 

 observe this staining property in the case of the hair-cells of 

 Tradescantia, where, it will be remembered, the nucleus becomes 

 deeply stained by the coloured cell-sap as soon as the cells are 

 killed by the action of alcohol. Other stains, again, affect the 

 cytoplasm rather than the nucleus, and these various chemical 

 reactions enable us to differentiate fairly sharply between the 

 different constituents of which the cell is composed, though it is 

 a matter of some doubt exactly how far the structure of the 



1 Greek XP"^ " a colour. 



