180 SURVEY OF CELL-LIFE.
formation of the cell-contents frequently follows, giving rise to a formation of new products in the cell-cavity. In most of the hollow cell-nuclei, the contents become paler, less granulous, and in some of them fat-globules, &c., are formed. (See pages 173, 4.) We may therefore say that the formation of cells is but a repetition around the nucleus of the same process by which the nucleus was formed around the nucleolus, the only difference being that the process is more intense and complete in the formation of cells than in that of nuclei.
According to the foregoing, then, the whole process of the formation of a cell consists in this, that a small corpuscle (the nucleolus) is the earliest formation, that a stratum (the nucleus) is first deposited around it, and then subsequently a second stratum (substance of the cell) around this again. The separate strata grow by the reception of new molecules between the existing ones, by intussusception, and we have here an illustration of the law, in deference to which the deposition takes place more vigorously in the external part of each stratum than it does in the internal, and more vigorously in the entire external stratum than in the internal. In obedience to this law it often happens that only the external part of each stratum becomes condensed into a membrane (membrane of the nucleus and membrane of the cell), and the external stratum becomes more perfectly developed to form a cell, than the nucleus does. When the nucleoli are hollow, which, according to Schleiden, is the case in some instances in plants, perhaps a threefold process of the kind takes place, so that the cell-membrane forms the third, the nucleus the second, and the nucleolus the first stratum. Probably merely a single stratum is formed around an immeasurably small corpuscle in the case of those cells which have no nuclei.
Varieties in the development of the cells in different tissues. Although, as we have just seen, the formative process of the cells is essentially the same throughout, and dependent upon a formation of one or many strata, and upon a growth of those strata by intussusception, the changes, on the other hand, which the cells, when once formed, undergo in the different tissues, are, in their phenomena at least, much more varied. They may be arranged in two classes according as the individuality
