STIMULI AND THEIR ACTIONS 367 



recognises a whole series of analogous cases in the various kinds of 

 pathogenic neoplasms or tumours, to which belong also malignant 

 cancers. These tumours (carcinoma, sarcoma, myoma, fibroma, 

 etc.) arise by the rapid division of the cells of a normal tissue, 

 e.g., the epidermis. There thus results in the particular place 

 an enormous increase of cells, a growth, which leads frequently 

 to a very extensive tumour and completely chokes the neigh- 

 bouring tissues in which it grows, so that they become incapable 

 of life and perish. Without doubt, in many cases this rapid 

 cell-increase is due to chemical causes acting upon the cells in 

 question. Although thus far it is an open question whether 

 or not tumours, especially carcinoma, are a result of infection 

 by certain micro-organisms, the majority of pathologists incline to 

 the view that they are to be traced to a change in the nutrition 

 of the cells. 



Much more evident than the effects of chemical stimuli upon 

 metabolism and form-changes are the effects upon tlie transformation 

 of energy, especially upon 

 movement. Kegarding 

 the effects upon the amce- 

 boid movements of naked 

 protoplasmic masses, such 

 as Bhizopoda, Amceha, 

 Myxomycetes, Polythala- 

 mict, and the protoplasmic 

 bodies of plant-cells, the 

 classic investigations of 

 Max Schultze ('63) and 



Kiihne ('64). over thirty Fig. \aS.—AmoAa. a, with pseudopodia extending in 



^ /'" f^ -> •% different directions. JJ, Creeping, with a long pseudo- 



yearS ago, nave atlOrded podium in one direction (form of Amceba liiaax). C, 



information. The most contracted to a ban upon chemical stimulation. 



wide-spread effect here is 



the calling-out of a contraction, i.e., the retraction of pseudopodia, 

 frequently after a preliminary acceleration of the protoplasmic 

 streaming at the beginning of the action. The greatest variety of 

 chemical substances can produce this reaction. If, e.g., to a drop of 

 water in which many amoebae exist there be added a 1 — 2 per cent, 

 solution of common salt, or a solution of 01 per cent, hydrochloric 

 acid, or of 1 per cent, potassium hydrate, or other acids, alkalies 

 and salts in weak solution, the amoebae immediately draw in their 

 pseudopodia and assume a spherical form (Fig. 158). Carbonic 

 acid exerts the same effect, if the amoebae be exposed in a gas 

 chember ^ for some time to the action of the gas. Other naked 

 protoplasmic masses behave similarly toward these chemical 

 stimuli. The delicate Adinosphcerium Eichhornii, which with its 

 straight, ray-like pseudopodia appears like a minute sun, when 



1 Cf. p. 283. 



