56 



GENERAL PRINCirLES OF ZOOLOGY 



mous with the division of the cell; but to understand this wc must first 

 consider the nucleus. This is a body enclosed in the protoplasm, whose 

 form, though definite for each kind of cell, shows in general wide varia- 

 tions. Usually it is spherical or oval, hut it may be elongated or rod- 

 shaped, bent into a horseshoe, with constrictions like a rosary, or even be 

 branched or treelike (fig. iS); in many living cells it is but little difierent 



Fig. iS. — ^'arious forms of nuclei, a, horscshoc-shajici! nucleus of an .\cinete; h, 

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

 nucleus of Stentor ccendcus. 



in appearance from the protoplasm and can only be seen with care and by 

 employment of a special technique based upon the microchemical reaction 

 of the nuclear substance. 



The Nuclear Substance. — The nuclear substance is distinguished 

 from protoplasm, among other ways, by its greater coagulability in certain 

 acids, e.g., acetic and chromic, which therefore are often used for demon- 

 strating the nucleus. In its minute structure the nucleus affords a wonder- 

 ful variety of pictures varying according to the objects chosen. Accord- 

 ing to their reactions to stains two substances in particular are distin- 

 guished: chromatin or nuclein (fig. 19, cJi), which is easily stained by certain 

 staining-fluids (carmine, ha;mato.xylon, satTranin), and the achromalin 

 or linin, which stains only vmder special conditions. 



The achromatin forms a network or reticulum (according to another 

 ■view a honeycomb structure) filled with a nuclear fluid, bounded exter- 

 nally by a nuclear membrane. If lillle nuclear lluid be present, and the 

 reticulum consequently be narrow-meshed, the nucleus seems compact. 



