PIGMENT.
The cells and the nuclei (the latter, however, in a much less proportion) increase in size, and at length those in the uppermost layers become flattened in such a manner that the nucleus forms the centre of the table. This, then, is but a repetition of the same course of development observed in most other cells. Before I had proved the universal accordance between animal and vegetable cells, Henle thought that the original increase in volume of the epithelial cells might possibly be explained as taking place by imbibition. (1. c., p. 9.) As, however, we have observed this growth to be a phenomenon which occurs in all animal cells—as we have seen the formation of cells around the nuclei—as a chemical change in the cell-membrane may be proved to take place during the expansion of many of the cells, and as it frequently happens that not only does no thinning of the cell-membrane occur during expansion, but that an actual thickening takes place, all which are processes similar to those of plants—we must ascribe a peculiar vitality to the animal as well as to the vegetable cells, and explain this expansion of the epithelial cells, like as we did that of plants, as a growth by intussusception. The new epithelial cells, it is true, are formed immediately upon the cutis only, where the greatest vital energy prevails; but the cells expand independently, and grow by intussusception. I have brought forward an instance in which a young epithelial cell was formed within another in the tadpole. But this is certainly a very rare circumstance, and the majority of epithelial cells, in all the vertebrate animals, are certainly not formed as cells within cells, but on the outside of the cells in a minimum of cytoblastema, which is exuded from the cutis. It might be objected that this process of formation of the epithelium could not be possible, for the reason that, if the cells of the second stratum were twice as large as those of the first, then the whole layer of epidermis must be also twice as large as the first. But this objection may be easily set aside by the fact that the cells slide upon one another, and a double or triple layer of cells may thus originate from one stratum of nuclei.
2. The Pigmentum nigrum. The pigment is familiarly known as being usually contained in round or (in consequence of their close apposition) hexagonal cells, in the form of innumerable
