GENERAL ANATOMY. 65 



it is disproportionally large, so that the protoplasm surrounds it 

 only with a thin layer, in others again it is so small that it can 

 scarcely be found in the protoplasm among the other substances. 

 Formerly, on this account, it was in very many cases overlooked, 

 and even now it can often be demonstrated only by great care, 



FIG. 18. Various forms of nuclei, a, horseshoe-shaped nucleus of an Acinete ; ibi 

 branching nucleus from the Malpighian vessel of a Sphingid larva ; c, rosary- 

 shaped nucleus of Stentor cceruleus. 



and by employment of a special technique based upon the micro- 

 chemical reaction of the nuclear substance. 



The Nuclear Substance. The nuclear substance is distin- 

 guished from protoplasm, among other ways, by its greater 

 coagulability in certain acids, e.g., acetic and chromic, which 

 therefore are often used for demonstrating the nucleus. If in a 

 living cell the nucleus be invisible on account of the similarity of 

 its refraction to that of the protoplasm, the addition of 2$ acetic 

 acid will often bring it into sharp contour. 



Structure of the Nucleus. In its minute structure the nucleus 

 affords a wonderful variety of pictures varying according to the 

 objects chosen, but which are not sufficiently understood to permit 

 of a single description accepted by all. According to their reac- 

 tions to stains two substances in particular are distinguished: 

 chromatin or nuclein (fig. 19, cli), which is easily stained by certain 

 staining-fluids (carmine, haematoxylon, saffranin), and the achroma- 

 tin or linin, which stains not at all or only under special conditions. 



The achromatin forms a network or reticulum (according to 

 another view a honeycomb structure) filled with a nuclear fluid, 



