242 THE SOLIDS. 



Valentin * confirms the accuracy of this description as 

 regards man. 



We find Henle f describing the epithelium of the ventricles 

 as a cuneiform ciliated epithelium ; and in Gerber's General 

 Anatomy, we remark that it is stated to be a tesselated 

 ciliated epithelium. It is singular how so great an error 

 could have originated, and still more so how it could have 

 been so long perpetuated. 



DEVELOPMENT AND MULTIPLICATION OF EPITHELIUM. 



Each epithelial cell is first detected as a nucleus without 

 any appearance of cell wall around it: this however may 

 exist closely embracing the nucleus, even from the earliest 

 period at which this can be observed. After a time, how- 

 ever, a transparent border becomes visible, surrounding 

 the nucleus : the width of this goes on gradually increasing 

 until at length the full dimensions of the cell have been 

 attained : now the outer limit of this border doubtless indi- 

 cates the cell wall, the clear space between it and the nucleus 

 being in the younger epithelial cells filled with fluid. In 

 the conical epithelium the cell wall is not developed equally 

 around the nucleus as in the pavement epithelium, but 

 chiefly in two opposite directions. 



It has been observed, that young epithelial cells are 

 thicker and rounder than the older ones ; it has also been 

 remarked, that they are more granular ; facts which stand in 

 close relation with their functional activity. 



It has been noticed likewise, that the nucleus in the pro- 

 gress of development becomes either obliterated, or that it 

 escapes from the cell. Now it has struck me that this dis- 

 appearance of the granular nucleus and granules of the cell 

 wall might possibly be connected with the reproduction 

 or multiplication of epithelial cells, as well as of cells occur- 

 ring elsewhere than in the epithelium, and that each granule 

 might in reality be an epithelial cell in embryo. This is 



* Repertorium, 1831, p. 158. 278. f Anat. Gen. t. vi. p. 253. 



