EPITHELIUM. 243 



of course but a conjecture : it is one, however, which would 

 appear not to be contradicted by any other known fact, and 

 to have analogy in its favour ; the mode of reproduction 

 amongst the lower algae being precisely similar. 



The occurrence of two nuclei in the same cell has been 

 recorded by some observers, and who have from this fact 

 drawn the inference, that epithelial cells are multiplied by 

 division. This method of increase cannot, however, be pre- 

 sumed to prevail to any extent, since it is a circumstance of 

 extreme rarity to meet with two nuclei in the same cell. 



NUTRITION OF EPITHELIUM. 



The epithelium has no immediate or structural connexion 

 with the parts which lie immediately beneath it, neither 

 does it receive blood-vessels and nerves from those parts ; it 

 is simply dependant upon them for the supply of nourish- 

 ment, of which it is the recipient, and which is derived from 

 the blood-vessels distributed throughout the tissue of the 

 true skin lying beneath it, and from which vessels the plasma 

 is continually escaping by transudation or exosmosis. 



DESTRUCTION AND RENEWAL OF EPITHELIUM. 



The epithelium in every part of the body is continually 

 undergoing a process of destruction, and consequent re- 

 newal. 



It is less easy to establish the fact of the destruction of 

 the epithelium in the closed cavities than in the open ; never- 

 theless, that it really does take place even in these, may be 

 inferred from the observation of the fact that the cells of 

 epithelium encountered in such localities represent every 

 degree of development, many of the older ones also being 

 destitute of nuclei, and more or less broken, into fragments. 

 It is probable, however, that the process of destruction is 

 slower in the closed than in the open cavities. 



