EPITHELIAL TISSUE. 33 



foundation to comparative histology in his remarkable 

 Lehrbuch der Histologie des Menschen und der Thiere 

 (Frankfurt, 1857), the study has been prosecuted with great 

 enthusiasm and success, and has been constantly stimulated 

 by improvements in microscopic apparatus and technique. 



We shall refer to interesting histological items in the 

 course of our studies, meanwhile we shall consider very 

 briefly the four great kinds of tissue ; — Epithelial, Con- 

 nective, Muscular, and Nervous. 



(a) Epithelial Tissue. 



In the development of many animals, the ovum divides and re-divides 

 till a hollow ball of cellTresults. The cavity is bounded by a layer of 

 similar cells, which often bear motile lashes or cilia on their outer ends. 

 Such a layer of cells illustrates what is meant by epithelium, and the 

 ■same simple kind of tissue composes all 'young embryos. Considered 

 ' embryologically, epithelium is the most primitive kind of tissue. 

 ' i Similarly, such an organism as Volvox, one of those which bridge the 

 gulf between single-celled and many-celled animals, consists of a ball of 

 cells not unlike the embryonic stage to which I have just referred. It 

 is a hollow sphere of epithelium. So Hydra is a tubular animal, with 

 ■two layers of cells, an external and an internal epithelium, in which 

 ^explications have just begun to arise. Considered historically, 

 epithelium is the most primitive kind of tissue. 



In- all epithelium which has not been much modified, the cells have 

 ' two'distinct poles. The outer pole is distinguished by cilia, or a sensitive 

 process, or a passive cutieular rim, or pigment, etc. , from the basal or 

 internal one. We can readily understand this, if we think of Volvox, 

 or of the hollow embryonic bajl, where the outer parts of the cell 

 obviously have a different environment from the internal ends. A great 

 many different kinds of tissue r^Sn- traces of epithelial origin and of 

 polar differentiations m their cells. But epithelial tissue has many 

 different forms in the more complex animals. The external layer of the 

 (Skih (epidermis), the internal lining of the food-canal and its out- 

 growths, the lining of the body-cavity, and the swathing of the organs 

 moored therein (endothelium), are all epithelial. Epithehum may be 

 single-layered or -stratified ; its cells may be columnar, scale-like, 

 or otherwise. The cells may be close together, or separated by inter- 

 cellular spaces, and they are often connected by bridges of living matter. 

 Nor are the fijnctions of epithelium less diverse than its forms, for it 

 may be ciliated (effecting locomotion, food-wafting, and respiration), or 

 sensitive (and as such forming sense-organs), or glandular (liberating 

 certain products or even the whole contents of its component cells), or 

 pigmented (and thus associated with respiration, excretion, and protec- 

 tion), or covered externally with a sweated-off cuticle, susceptible of 

 many modifications (especially of protective value). 



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