20 GENERAL HISTOLOGY. 



of threads, knobs, fringes, or other projections, so that the 

 same form is but momentary. Of course this change is only 

 possible to free bioplasts. 



(3) In wandering movement a thread or arm is projected. 

 from the mass, along which, as a sort of bridge, the semi-fluid 

 material flovvs and accumulates at the further end. Thus bio- 

 plasts move or wander from place to place. Such motions 

 may be seen in the mesenteric vessels of a frog under the 

 microscope, the white blood cells attaching themselves to the 

 walls of the vessel, then penetrating the wall, and passing 

 through the tissues outside. The stimuli of gentle heat, light, 

 or electric currents accelerate all these movements, while cold 

 or excessive heat, etc., retards them. They, also respond to 

 mechanical stimuli. 



2. Nutrition is the appropriation of new material molecules 

 to replace the old ones as they are changed into tissues or be- 

 come effete. 



The chemical composition of the tissues differs from that of 

 the blood, and still more from the food. There is a decompo- 

 sition of the pabulum, a selection of appropriate molecules 

 and a rearrangement of such molecules in the bioplasm, quite 

 unlike the chemical transformation of the non-living. It dif- 

 fers from crystallization, which is a deposit along certain axes of 

 attraction of a similar substance in solution, which after crys- 

 tallization may be dissolved and recrystallized, while bioplasm 

 is nourished from dissimilar substances. It differs also from 

 catalysis, or the phenomena induced by the presence and in* 

 fluence of a third material substance, as when spongy platinum 

 induces O and H to combine. In nutrition there is no known 

 third substance present, but the bioplasm itself selects nutrient 

 chemical elements from the pabulum, dissolving their former 

 affinities, and recombining them after the laws of a vital chem- 

 istry. We distinguish between nutrition of bioplasm and the 

 building of tissues, although nutrition may have reference to 

 the nature of the tissues to be formed. 



3. Growth is the increase of bioplasm within normal limits- 

 Non-living matter, amorphous or crystalline, enlarges by 



accretion, or deposit of similar matter on the outside, but bio- 



