GENERAL ANATOMY 55 



microscopes (fig. 17), yet in them the granules wander hither and thither 

 nice people on a promenade, simultaneously ccntripetally and centrif ugally, 

 some with greater, others with less speed. The granules are moved by 

 the protoplasm, for if we feed the creature with finely-pulverized car- 

 mine, these granules show the same remarkable streaming. Indeed, 

 nothing better illustrates the great complexity of protoplasm than these 

 phenomena of motion in such narrow limits as pseudopodia in general. 



Irritability of Protoplasm. — That amceljoid movements and 

 streaming of granules can be induced, brought to a standstill, and modified 

 by mechanical, chemical, and thermal stimuli, is a proof of the irritability 

 of protoplasm. Tvlost important are the thermal stimuli; if the surround- 

 ing medium rise above the ordinary temperature, the movements at first 

 become more rapid up to a maximum: from that point begins a slowing, 

 finally coming to a standstill — heat-rigor. If the high temperature 

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

 temperature for most animals is between 40° and 50° C. (io4°-i22° F.); 

 its influence explains a part of the injurious effects which high- 

 fever temperatures have upon the human organism. Like the heat- 

 rigor, there is a cold-rigor, induced by a marked sinking of the tempera- 

 ture 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, 

 consequently 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° C; frogs, to - 28°; 'blind worms', to - 25°). 



Nutrition and Reproduction. — Irritability and power of motion 

 are necessary for assimilation. Most animal cells, for example almost 

 all the tissue cells, are not suitabe for studying assimilation, because they 

 live upon liquid nourishment. But certain cells of higher animals, 

 the leucocytes, and most unicellular animals can be fed with solid 

 substances; they take the food-particles into the midst of the protoplasm 

 by flowing around them with the pseudopodia. They extract all the 

 assimilable and reject the indigestible portions (fig. 16). 



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 power of producing 

 substances other than protoplasm; for example, many Protozoa form 

 shells or skeletons which are hardened with silica or lime. This formative 

 power, the building of plasviic products, is the starting-point for tissue- 

 formation. 



Cell Nucleus. — The reproduction of protoplasmic bodies is synony- 



