STRUCTURE OF LIVING MATTER 5 



changes of reaction occur when the living substance dies, and slight 

 changes of reaction, i.e., changes in the relative concentration of 

 hydrogen ions (H + ) and hydroxyl ions (OH - ) , can bring about 

 similar precipitates in colloid solutions. Nevertheless in some cells 

 a certain differentiation in the structure of the protoplasm can be 

 seen during life and before the addition of any reagent, and in such 

 cases there can be no doubt that the structural details pre-exist 

 and are not arte-facts. In certain respects protoplasm behaves 

 like a liquid, and in others like a solid, a peculiarity which is un- 

 doubtedly associated with the fact that its chief constituents exist 

 in the colloid state, as experiments with such substances as gelatin 

 and agar have shown. In building up our typical cell we start with 

 a piece of protoplasm. Somewhere in the midst of this we find a 

 body which, if not absolutely different in kind from the protoplasm 

 of the rest of the cell or cytoplasm, is yet marked off from it by very 

 definite morphological and chemical characters. 



This is the nucleus, generally of round or oval shape, and bounded 

 by an envelope. Within the envelope lies a second network of 

 fine threads, which do not themselves stain with nuclear dyes such 

 as hsematoxylin. But in or on these ' achromatic ' filaments lie 

 small, highly .refractive particles, staining readily and deeply with 

 dyes, and therefore described as consisting of chromatin. This chro- 

 matin is either made up of nucleins (conjugated proteins particu- 

 larly rich in nucleic acid, and therefore in phosphorus), or yields 

 nucleins by its decomposition; and it seems to owe its affinity for 

 certain staining substances to the presence of nucleic acid. The 

 meshes of the nuclear reticulum contain a semi-fluid material, 

 which does not readily stain. The nucleus is distinguished from 

 the cytoplasm, even as regards its inorganic constituents, by the 

 absence of potassium.* Besides the nucleus, another much smaller 

 structure, the centrosome, is differentiated from the protoplasm 

 of many cells. This is a minute dot staining deeply with such dyes 

 as haematoxylin, and generally situated near the nucleus. Sur- 

 rounding it is a clear area, the attraction sphere, in and beyond 

 which fine fibrils radiate out into the cytoplasm. Both the attrac- 

 tion sphere and the nucleus play an important part in division of 

 the cell by the process known as karyokinesis, or mitosis, or in- 

 direct division, which is by far the most common mode. 



When the nucleus is about to divide, the chromatin granules 

 arrange themselves into one or more coiled filaments or skeins, 

 which then break up into a number of separate portions called 



* This has been shown microchemically. The potassium is precipitated 

 by a solution of hexanitrite of sodium and cobalt as orange-yellow crystals of 

 the triple salt, hexanitrite of potassium, sodium, and cobalt. Where very 

 minute traces of potassium are present, ammonium sulphide must be added, 

 after washing out the excess of the cobalt reagent. Black cobalt sulphide is 

 thus formed from the triple salt (Macallum, Frontispiece). 



