PIGMENT. 77 
The cells and the nuclei (the latter, however, in a much less 
proportion) increase in size, and at length those in the upper- 
most 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, ike 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 epi- 
thelium 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 origimate 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 
