10 THE ANIMAL CELL [CH. II. 



structure would be invisible. If such dyes as haematoxylin or 

 safranin are employed, it is the nucleus which becomes most deeply 

 stained, and thus stands out on the lighter background of the 

 protoplasm. 



But the whole nucleus does not stain equally deeply ; it is the 

 chromoplasmic filaments and the nucleoli which have most affinity for 

 the stain, while the nuclear sap is comparatively unaffected. Hence 

 the terms chromatin and achromatin originally introduced by Fleming. 

 The membrane, the network, and the nucleoli are composed of chro- 

 matic substance or chromatin ; it is so called not because it has any 

 colour in the natural state, but because it has an affinity for colours 

 artificially added to it. For a corresponding reason, achromatin or 

 achromatic substance is the name given to the 

 substances which make up the nuclear sap. 



Balbiani showed that the chromoplasmic filaments are 

 apparently transversely marked into alternate dark and light 

 bands ; this is due to the existence of minute highly refracting 

 particles imbedded in regular series in a clear homogeneous 

 and unstainable matrix (see fig. 7). The term chromatin should 

 properly be restricted to these particles. These particles have 

 special affinity for basic dyes, such as methylene blue and 

 safranin. 



Coming next to the chemical composition of the 

 nucleus, it is found to consist principally of protein 

 ma* an d protein-like substances. The nuclei of cells 

 ment, greatly magni- ma y \) Q obtained by subjecting the cells to the 



tied. (Oarnoy.) * p i -i i it j i 



action of artificial gastric juice; the protoplasm is 

 nearly entirely dissolved, but the nuclei resist the solvent action of 

 the juice. No doubt the nuclei contain several chemical compounds, 

 but the only one of which we have any accurate knowledge has 

 been termed nuclein, and this is identical with the substance called 

 chromatin by histologists. It is soluble in alkalis,, but precipitated 

 by acids; it is different from a simple protein, as it contains in addi- 

 tion to carbon, nitrogen, oxygen, hydrogen, and sulphur, a large 

 quantity (7 to 8 per cent, or even more) of phosphorus in its 

 molecule. In many cases nucleins contain iron also. 



The Attraction Sphere. 



In addition to the nucleus and protoplasm, all living cells 

 contain another structure, namely, a minute particle called a 

 " centrosome" which has an attractive influence on protoplasmic 

 fibrils and granules in its neighbourhood, the whole appear- 

 ance produced being called an attraction sphere (fig. 8). 



It is most prominent in cells which are dividing or about to 



