74 EPITHELIUM. 



their being flattened. The serous layer of the germinal mem- 

 brane also cannot well be considered to be epithelium, although 

 it has the same structure, and yet it is difficult to give a defini- 

 tion of it which shall not comprise these structures. We shall 

 not, however, enter upon this contention about mere terms, but 

 proceed to the consideration of the structure of the epithelium. 

 The simplest form of epithelium is that of the round cells 

 furnished with a nucleus which lies upon the inner surface of 

 their wall, and encloses one or two nucleoli. When in con- 

 nexion they assume a polyhedral form, but their free surface 

 usually projects in the form of a, segment of a sphere. Such 

 is the appearance presented by the epithelium in many situa- 

 tions ; I instance only that of the branchial rays of the fish 

 by way of illustration. The cells are usually smaller and more 

 granulous in mammalia ; but in the lower animals and in the 

 foetal stage of mammalia they are, in general, larger, smoother, 

 and sometimes so transparent as to be visible by a subdued 

 light only. I once had an excellent opportunity of observing 

 the epithelium upon the mucous membrane of the stomach of 

 a fcetal sheep, and its perfect resemblance to the parenchy- 

 matous cellular tissue of plants. A minutely granulous deposit 

 may often be observed in the interior of the transparent epi- 

 thelial cells ; in those of the branchial rays of the fish, for 

 instance, it appears to be formed in the neighbourhood of the 

 nucleus. According to Henle, two nuclei never occur in an 

 epithelial cell in mammalia ; but I have several times observed 

 that number in the external covering of the tadpole, and on one 

 occasion I remarked that a perfectly developed epithelial cell 

 furnished with a nucleus was enclosed within a larger cell. 

 Changes in form from this rudimentary globular shape occur in 

 the epithelial cells in two different manners ; they either become 

 flattened into tables, or prolonged into cylinders. The flattening 

 out into tables takes place in such a manner that the nucleus 

 forms the centre of one surface, as in the blood-corpuscle. I 

 have observed the stages of transition from the globular to the 

 tabular form in the epithelium of the external covering of the 

 tadpole, which occasionally presented hexagonal flat columns 

 or tables, the thickness of which was about equal to one third 

 of their breadth. The thickness is so very slight in proportion 

 to the breadth in the completely flattened epithelial cells, that 



