INORGANIC SALTS 173 



In explanation of these apparent discrepancies it 

 has been suggested that smooth muscle cells are easily 

 injured and that the normal semi-permeability had dis- 

 appeared at the time when the tissue was examined, but 

 this possibility is rejected by Meigs. It is difficult to 

 escape the conclusion that a continuous semi-permeable 

 membrane does not invest the whole cellular element in 

 this tissue. Possibly the contractile part of the entire 

 structure has a relation to the protoplasmic part similar 

 to that which the fibers in connective tissue have to the 

 cells by which they are formed. The sluggishness of 

 the movement and its slow reversibility suggest a 

 fundamentally different type of organization from 

 that of striated muscle cells. Semi-permeability appears 

 to be universal in indifferentiated cellular elements and 

 in the majority of specialized cells; but a differentiation 

 in which part of the total structure acquires non-cellular 

 properties is not infrequent in organisms, as illustrated 

 in connective tissues and skeletal structures; and the 

 above-cited types of contractile tissue may exemplify 

 this general condition. 



The penetration of salts through the egg-membrane 

 of the sea-minnow, Fundulus, as shown in various toxic 

 effects and antagonisms, illustrates many conditions of 

 great interest, and especially indicates the importance 

 of the purely chemical factors in permeability. This 

 membrane is a dead structure, or chorion, external to 

 the living protoplasm of the egg, and apparently consist- 

 ing chiefly of a keratin-like protein. In its normal 

 state, it is almost completely impermeable to water or 

 the salts of the medium; hence the eggs will develop 

 either in distilled water or in concentrated sea water. 



