PIGMENT. 



/ • 



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, 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 cvtoblastema, 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 originate from one stratum 

 of nuclei. 



2. The Pif/mentum 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 



