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 
eranulous 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 foetal 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; im 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 | 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 m proportion 
to the breadth in the completely flattened epithehal cells, that 
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