7 GENERAL PRINCIPLES OF ZOOLOGY. 



that the cells use for their own growth and for replacing 

 worn-out parts the food which they have taken up, but 

 also that most of them have the power of producing sub- 

 stances other than protoplasm, as, for example, many pro- 

 tozoa form organic aggregates which are hardened with 

 silica or chalk. This formative power, the building of 

 "protoplasmic products,'" is, as we shall shortly see, the 

 starting-point for tissue-formation. 



a 



FIG. 18. Various forms of nuclei, a, horseshoe-shaped nucleus of an Acinete: t, branch- 

 ing nucleus from the Malpighian vessel of a Sphingidie larva; c, rosary-shaped nucleus of 

 Stentor cceruleus. 



Cell Nucleus. - The reproduction of protoplasmic 

 bodies is synonymous with the division of the cell ; but to 

 understand this we must first consider the second important 

 constituent, the nucleus. 



The cell nucleus is a body enclosed in the protoplasm, 

 whose form, though definite for each kind of cell, shows in. 

 other respects wide variations. Usually it is a spherical or 

 oval vesicle ; but it may be elongated or club-shaped, 

 bent into a horseshoe, with constrictions like a rosary, or 

 even be branched, treelike (Fig. 18); in many cells 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 and surrounding 



