64 GENERAL PRINCIPLES OF ZOOLOGY. 



ordinary temperature, the movements at first become more rajjid 

 w]} to a maximum: from that point begins a slowing, final]}' coming 

 to a standstill. — Ihat-rigor. If the high temperature continue 

 much longer, or if it rise still higher, death results. The fatal 

 temperature is found for most animals between 40""^ and 50° C. 

 (1()4"_122'' F.); its influence ex])lains a part of the injurious 

 effects which high-fever temj)eratures have upon the human 

 organism. Like the heat-rigor, there is also a, cold-rigor, induced 

 by a sharp sinking of the temperature below the normal. This 

 is accompanied by a gradual diminution of mobility; it results 

 in death by freezing, which is, however, not so easily produced as 

 death by heat. It is a remarkable fact that many animals, conse- 

 quently their cells, may be frozen; and in this condition can 

 endure still severer cold without dying. (For example : goldfish, 

 a temperature of — 8° to —15° Q.\ frogs, to — 28°; newts, to 

 - 25°). 



Nutrition and Reproduction. — Irritability and power of motion 

 are the prerequisites of assimilation, the change of food-substance 

 into protoplasm. Most animal cells, for example almost all tissues 

 cells, are not suitable for studying assimilation, because they live 

 uj)on liquid nourishment. But certain cells of higher animals, 

 the colorless blood -cells, and most unicellular animals can be fed 

 also with solid substances; they take the food-particles into the 

 midst of the protoplasm by flowing around them with the pseudo- 

 l^odia. They extract all the assimilable and reject the indigestible 

 portions (fig. 10). 



In the case of assimilation it is to be noted not only that the 

 cells use the food which they have taken for their own growth and 

 for replacing worn-out parts, but also that most of them have the 

 250wer of producing substances other than protoplasm; for 

 example, many Protozoa form organic shells or skeletons which 

 are hardened with silica or lime. This formative power, the 

 building of ' plasmic ])7'odvcts,' is, as we shall shortly see, the 

 starting-point for tissue-formation. 



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. This is a bodj^ enclosed in the protoplasm, whose form, 

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

 tions. 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 



