50 



GENERAL PRINCIPLES OF ZOOLOGY 



mous with the division of the cell; but to understand this we 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, but it may be elongated or rod- 

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

 branched or treelike (fig. 18) ; in many living cells it is but little different 



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

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

 nucleus of Stentor cceruleus. 



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 nude in (fig. 19, ch), which is easily stained by certain 

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

 or linin, which stains only under 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 little nuclear fluid be present, and the 

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



