GENERAL ANATOMY. 77 



extremely complicated phenomena of motion in such nar- 

 row limits as pseudopodia in general. 



Irritability of Protoplasm. That amoeboid movements 

 and streaming of granules can be induced, brought to a 

 standstill, and modified by mechanical, chemical, and ther- 

 mal stimuli, is a sure proof of the irritability of protoplasm. 

 Most important are the thermal stimuli ; if the surrounding 

 medium rises above the ordinary temperature, the move- 

 ments 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 continues much 

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

 temperature is found for most animals between 40 and 50 

 Celsius (104 122 F.) ; its influence explains the injurious 

 effects which high-fever temperatures have upon the human 

 organism. 



Cold-rigor. - -Like the heat-rigor, so 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 induced as death by heat- 

 ing. 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 Cent. ; frogs, to 

 - 28; newts, to -- 25.) 



Nutrition and Reproduction. Irritability and power 

 of motion are the prerequisites of assimilation, the change 

 of food-substances into protoplasm. Most animal cells, as, 

 for example, almost all tissue cells, are not suitable for 

 studying assimilation, because they live upon liquid nour- 

 ishment. 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 

 pseudopodia. They withdraw all the assimilable and re- 

 ject the indigestible portions (Fig. 16). 



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



