DEVELOPMENT OF CELLS. 353 



Sometimes the spaces between cells have no intercellular substance, 

 there may be instead fluid or air ; such cavities may present different 

 forms they are termed intercellular passages, and are for the con- 

 veyance of fluid or the passage of secretions : they exist in animals 

 and vegetables, but are more highly developed in animals. In vege- 

 tables there are other cells lining these intercellular passages, so as to 

 form a regular tube, with walls consisting of different coats. Cells 

 exist in the neighbourhood of these passages, and have shot out pro- 

 cesses and coalesced to form vessels with similar offsets from other 

 cells : ultimately these are seen to join the intercellular passages, and 

 become continuous with them. In glands, the cells being filled with 

 their peculiar fluid, are conveyed to the wall of the intercellular pas- 

 sage, and through this the secretion arrives at the surface of the body. 



The Epithelium. If we cut very thin slices from the superficial 

 portions of the skin, we can raise from it a delicate membrane ; or what 

 is better, by using chemical or mechanical irritation, we obtain what is 

 ordinarily called a blister : to it we give the name of epidermis. The mi- 

 croscope has shown this to be a tissue of high and remarkable organisa- 

 tion, being, in point of fact, an aggregation of laminated cells, differing, 

 in different situations, in regard to form, colour, and composition. This 

 investment serves to protect the delicate structures beneath, and is 

 likewise a bad conductor of heat, thus tending to maintain the tempe- 

 rature of the body ; besides these uses, it answers the purposes of ex- 

 cretion, and is sometimes an agent of motion. These laminated elemen- 

 tary cells, found on the surfaces, have generally nuclei. The nucleus is 

 rounded or of oval form, and is the 1-3000 to 1-5000 of an inch in dia- 

 meter. Each nucleus has two or three nucleoli, with outlines more or 

 less irregular; a cell surrounds the whole, which has transparent walls. 

 The cell varies in this latter arrangement : it may be flattened, and 

 the nucleus maybe attached to one side of it; or again, the nucleus 

 may be in the centre, and the cell prolonged at either end. The cells 

 of the epithelium may be divided into three kinds : the 1st is termed 

 the tesselated or pavement; 2d, the columnar or basaltic; 3d, the 

 ciliated or vibratile epithelium. Some make a 4th, combining the tes- 

 selated and columnar : this may be considered as transition epithelium, 

 and is found only in certain mucous passages. These cells are repre- 

 sented in Plate XII., figs. 1, 2, and 3. 



Tesselated epithelium is the simplest form, and, as its name 

 implies, resembles flags of pavement, overlapping each other at 

 their edges. They assume more or less the polygonal form, and 

 their size varies in the different serous membranes. The cells of the 



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