CH. II.] 



THE NUCLEUS 



11 



FIG. 8. Diagram of nucleus showing the 

 arrangement of chief chromatic filaments. 

 Viewed from the side, the polar end being 

 uppermost. p.c.f., primary chromatic fila- 

 ments; 7?.,nucleolus ; n.o.m., node of mesh- 

 work. (Rabl.) 



right-hand side of the figure). This observer also supposes that the 

 primary fibres have the looped arrangement depicted in the diagram. 



In the investigation of microscopic objects, a histologist is nearly 

 always obliged to use staining agents ; the extremely thin objects he 

 examines are so transparent that, without such stains, much of the 

 structure would be invisible. If 

 such dyes as hsematoxylin or 

 safranin are employed, it is* the 

 nucleus which becomes most deeply 

 stained, and thus stands out on the 

 lighter background of the proto- 

 plasm. 



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 achro- 

 matin originally introduced by Fleming. The membrane, the net- 

 work, and the nucleoli are composed of chromatic 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 achro- 

 matic 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. 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 

 Fl c G hromopiasmic f fii a a may be obtained by subjecting the cells to the 

 ment, greatly magni- action of artificial gastric juice; the protoplasm is 



fled. (Carnoy.) . .. J I r < 1 t 



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 accu- 

 rate 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 sul- 

 phur, an enormous quantity (7 to 8 per cent, or even more) of phos- 

 phorus in its molecule. In many cases nucleins contain iron also. 



