76 EPITHELIUM. 



stitute the other modification in the form of the epithelial cells. 

 They were discovered by Henle in the intestinal mucus-mem- 

 brane. Thev likewise enclose the characteristic nucleus, and are 

 arranged with their longest sides in apposition. Their blunt 

 ends are turned outwards and free. The opposite end either 

 terminates abruptly also, as in the chorion, or proceeds to a 

 point. This tapering figure frequently commences at the upper 

 part, so that the cells then have the form of a pointed cone, 

 the base of which is turned towards the outside. Henle found 

 that the cilia stand upon the free surfaces of the epithelial j 

 cylinders in those membranes which present the phenomenon of 

 ciliary motion, a fact of itself sufficient to show that the epithe- 

 lium ought not to be regarded as a mere inanimate covering 

 to the organized structures. 



With regard to the formation of the epithelial cells, Henle 

 has already proved the rete Malpighii to consist of round 

 nucleated cells, probably the young epidermal cells, and also 

 that the diameter of the cells increases towards the outside, so 

 that in the fcetal pig he was enabled to trace the gradual transi- 

 tion of the cells of the rete Malpighii into those of the epidermis. 

 (Symbolse ad anatomiam villor. intest., p. 5.) An actual growth 

 of the epithelial cells thus became very probable ; I have likewise 

 followed this process in the fcetal pig. The uppermost layer of 

 the epidermis is there formed of large, tabular, hexagonal cells, 

 furnished with a nucleus. Immediately beneath these lie 

 nucleated cells, which are already much smaller, and almost 

 round, so that the flattening must take place very rapidly. The 

 farther you proceed from the surface the smaller the cells be- 

 come, and the closer they encompass the nucleus. The size 

 of the nucleus also diminishes in some degree, but by no means 

 in the same proportion. In the lowest strata, the cells cannot 

 any longer be distinguished, but the nuclei lie close together, 

 with a small quantity of minutely granulous intermediate sub- 

 stance. It is, however, very difficult to obtain positive convic- ■ 

 tion of this fact, for the stratum of nuclei is too firmlv connected ! 

 with the cutis. We shall have an opportunity of observing 

 this relation of the nuclei more distinctly hereafter in the 

 feather. The mode of formation is probably this : cell-nuclei are 

 formed, in the first place, immediately upon the surface of the 

 cutis; and then around, and closely encompassing them, the cells. 



