182 SURVEY OF CELL-LIFE. 



cover the internal surface of the cell-membrane, or not be in 

 contact with each other at all. I have not observed any such 

 secondary stratified depositions in animals. 



The variations which may occur in the growth of the cell- 

 membrane in simple cells, depend upon the circumstance as to 

 whether or not the addition of new molecules takes place 

 equably at all parts of the cell-membrane. In the first case 

 the form of the cell remains unchanged, and the only other 

 distinction possible would be grounded upon the fact as to 

 whether the greater part of the new molecules were deposited 

 between the particles which lay side by side upon the super- 

 ficies of the cell-membrane, or between those which lay one 

 behind another in its thickness. The first mode of growth 

 produces an expansion of the cell-membrane, the effect of the 

 second is more especially to thicken it. Both modes are gene- 

 rally combined, but in such a manner that the expansion of the 

 cell-membrane prevails in most instances. 



A great variety of modifications in the form of the cells may 

 be produced by the irregular distribution of the new mole- 

 cules. The globular, which is their fundamental form, may 

 be converted into a polyhedral figure, or the cells may become 

 flattened into a round or oval or angular tablet, or the expan- 

 sion of the cells may take place on one or on two opposite sides, 

 so as to form a fibre, and these fibres again may either be flat, 

 being at the same time in some instances serrated, or lastly, 

 the expansion of the cells into fibres may take place on dif- 

 ferent sides so as to give them the stellate form. Some of 

 these changes of form are, no doubt, due to mechanical causes. 

 Thus, for example, the polyhedral form is produced by the 

 close crowding of the spherical cells, and these, when separated 

 from one another, sometimes assume their round figure again; 

 such is the case with the velk-cells. Some of the other 

 changes would seem to be capable of explanation by exosmosis. 

 If, for example, the contents of a round cell be so changed, 

 that a fluid is generated in it which is less dense than the 

 surrounding fluid, the cell will lose some of its contents by 

 exosmosis, and must, therefore, collapse, and may become 

 flattened into a table as in the blood-corpuscles. Such expla- 

 nations, however, are unsatisfactory in by far the greatest num- 

 ber of instances, and we are compelled to assume, that the 



