CH. II.] 



THE NUCLEUS. 



II 



Fig. 8. Diagram of nucleus showing the arrange- 

 ment of chief chromatic filaments. Viewed 

 from the side, the polar end being uppermost. 

 p.c.f., primary chromatic filaments ; n., nu- 

 cleolus; n.o.m., node of mesh work. (Rabl.) 



haematoxylin or safraniu 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 

 chromolpasmic filaments and the nucleoli which have most 

 affinity for the stain, while the nuclear membrane and the 

 nuclear sap are comparatively unaffected. Hence the terms 

 chromatin and achromatin 

 originally introduced by 

 Fleming. The network 

 and the nucleoli are com- 

 posed of chromatic sub- 

 stance or chromatin ; it is 

 RO 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 membrane and nuclear sap. 



To these general facts one or two details may be added. Balbiani first 

 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. 9). The term chromatin 

 should properly be restricted to these particles. These 

 particles have special affinity for basic dyes like methyl 

 green, and safranin. 



Coming next to the chemical composition of the 

 nucleus, it is found to consist principally of proteid 

 and proteid-like substances. The nuclei of cells 

 may be obtained by subjecting the cells to the 

 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 proteid, as it contains in addition to carbon, 

 nitrogen, oxygen, hydrogen and sulphur, an enormous quantity 

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

 In many cases nucleins contain iron also. 



Fig. 9. Part of 

 a chromo- 

 plasmic fila- 

 ini-nt, greatly 

 magnified. 

 (Carnoy.) 



