INTRODUCTION 5 



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 net- 

 work 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 chromatin is either made up of nucleins (nucleo- 

 proteins particularly rich in nucleic acid, and therefore in phos- 

 phorus), 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 the cell. This is a minute dot staining 

 deeply with such dyes as haematoxylin, and generally situated 

 near the nucleus. Surrounding it is a clear area, the attraction 

 sphere, in and beyond which fine fibrils radiate out into the 

 cytoplasm. Both the attraction sphere and the nucleus play an 

 important part in division of the cell by the process known as 

 karyokinesis, or mitosis, or indirect 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 

 chromosomes. These undergo a remarkable series of transforma- 

 tions, leading eventually to the segregation of the nuclear 

 chromatin in two separate daughter nuclei, each surrounded by 

 a portion of the original cytoplasm. Apart from its role in the 

 division, and therefore in the multiplication, of the cell, the 

 nucleus is now known to exert an influence perhaps not less 

 important upon those chemical changes in the cytoplasm which 

 are necessary for its normal nutrition and function, j" It is doubt- 

 ful whether any portion of protoplasm can permanently survive 



* 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 sa.lt (Macallum, Frontispiece). 



f According to Hertwig, a precursor of chromatin, ' prochromatin/ a 

 substance without characteristic staining reaction, is formed in the cyto- 

 plasm, taken up by the nucleus, and there elaborated into chromatin. 

 From the nucleus chromatin and its derivatives return to the cytoplasm 

 to be used in its function. 



