TISSUE FORMATION. 53 



and certain properties called osmose and molecular coales- 

 cence, all combine, so as to render the study of some 

 tissues quite complex. In other cases the mode of for- 

 mation is readily traced. 



3. The action of physical stimuli, as heat, etc., upon 

 bioplasm itself is yet very imperfectly known. Light is 

 not essential to its development, as is seen in the growth 

 of fungi, the cells of the interior of organisms, and of the 

 embryo in the dark. Many experiments on bioplasm 

 have shown that a moderate increase of temperature 

 quickens its movements, and a corresponding depression 

 retards them. Electrical, mechanical, and chemical stim- 

 ulation have similar effects to heat. Yet the action of 

 these stimuli vary in different cases. The motions of 

 amoebae are arrested by iced water, and recommence on 

 raising the temperature, yet the segmentation of trouts' 

 eggs proceeds well in iced water, but in a warm room they 

 soon die.* If the change of intensity in the stimulation 

 be made gradually, and not suddenly, the living matter 

 will sometimes adapt itself to it without serious disturb- 

 ance. Animals have been frozen and revived, and there 

 are instances on record of men enduring for a consider- 

 able time without much inconvenience the heat of ovens 

 raised to 500 F. 



The influence of light, heat, and electricity upon formed 

 material of different kinds is very great, but the com- 

 plexity of the organism and of the phenomena render it 

 difficult to know what part is supplied by the bioplasm 

 and what by its product. The vegetable bioplasm of 

 the interior grows and reproduces its kind, but the 



* Strieker's " Manual of Histology." 



