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 superficies 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 molecules. 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 expansion 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 different 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 yelk-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 explanations, however, are unsatisfactory in by far the greatest number of instances, and we are compelled to assume, that the
