20 THE CELL 



Water is an integral constituent of living substance and on drying the 

 cell either dies or it becomes apparently dead ("dry rigor"), and resumes 

 its vital activities again on the addition of water. 



We do not know anything definite about the manner in which water is 

 combined with the real living substance. That it is not held in pores of the 

 protoplasm as in a sponge appears from the fact that water cannot be pressed 

 out by mechanical means. No more can the water here be regarded as an 

 analogue of the water of crystallization in inorganic salts. It seems more likely 

 that it is held in interstices between the molecules or combinations of molecules 

 which make up the living substance. It is not impossible also that at the death 

 of the protoplasm a part of the water constitutes a product of disintegration. 



The specific gravity of protoplasm is somewhat greater than that of water. 

 It refracts light more strongly than water, is transparent in thin layers, 

 opaque in thick layers. Some forms of living matter are doubly refractive. 

 This property was first observed in cross-striped muscle (Boeck) ; but since 

 that time it has been found that practically all contractile substance differ- 

 entiated into fibers, such as smooth muscle cells, cilia, etc., is positively doubly 

 refractive in such a way that the optical axis coincides with the direction of 

 the fibers (Engelmann). This fact is evidence that the structures in ques- 

 tion have a different molecular arrangement from that of other living 

 structures. 



We know nothing concerning the chemical constitution of living substance. 

 Chemical investigation of dead animal and plant bodies has made us ac- 

 quainted with a very large number of different organic and inorganic com- 

 pounds : but not even the delicate micro-chemical reactions have been able to 

 furnish any information on the chemical nature of living substance. We can 

 only say, therefore, that when the living substance dies we are able to demon- 

 strate proteid bodies of different kinds as the chief constituents, and that 

 in animals at least the living substance can be formed, as it appears, only from 

 proteid bodies (cf. Chapter III). 



D. MORPHOLOGY OF THE CELL CONTENTS 



Everywhere, in plants as well as in animals, protoplasm has the same 

 appearance, just as it is everywhere essentially the same with respect to its 

 fundamental vital properties. Even with the highest possible magnification 

 we are unable to distinguish the protoplasm of a plant cell from that of an 

 animal cell. This similarity is of course only apparent, for, since the life 

 process in every particular organism takes place in a way peculiar to itself, 

 and since the protoplasm outside the nucleus represents the theater of 

 different vital activities, these outstanding differences must be conditioned by 

 a difference in the quality of the protoplasm (0. Hertwig). 



Leaving out of account the apparent similarity of the protoplasm, different 

 cells may as a whole present a very different appearance. This is due partly 

 to the external form of the cell and its envelope, which must be regarded as 

 something secondary at least, partly to differentiations inside the cell (cilia, 



