THE TISSUES. 



21 



the Vegetable Kingdom ; whilst in that kingdom the animal tissues, 

 which are the source of the arrangements characteristic of animals, 

 are wanting. All other tissues, though often distinguished by name, 

 are either not independent tissues at all, but only much more 

 complicated structures formed of a variety of tissues, or are forms of 

 tissue which should be ranged under one of the above-mentioned 

 categories, or may be merely component parts of the tissues already 

 named. We overlook the true conception of what a tissue is if we call 

 structures which are made up of several tissues " compound tissues." 



A. Vegetative Tissues. 

 Epithelium. 



§ 18. 



Cells placed side by side, and forming one or more layers which 

 invest the surface of the body or the walls of internal spaces, are 

 called epithelial. Epithelial tissue, then, consists simply of cells. 

 It is distinguished from other tissues by the fact that the cells, at 

 least so far as their arrangement is concerned, retain their primitive 

 characters. Epithelium represents phylogenetically, and therefore, 

 also, ontogenetically, the oldest form of tissue. The germinal 

 layers which are the earliest organological products of the differ- 

 entiation of the masses of cells which arise from the segmentation 

 of the egg-cell, are layers of epithelial cells. Epithelial cells vary 

 greatly in form, and are the starting-point of various 

 organs. The protoplasm of epithelial cells very often 

 loses its homogeneous character, owing to the differen- 

 tiation of its outermost layer into a thickened membrane. 

 In stratified epithelium this is best seen in the superficial 

 layers, the absence of a membrane in the cells of the 

 deeper layers being an indication of their younger con- 

 dition. 



Another differentiation is the development by the 

 superficial layer of cells on the surface exposed to the ex- 

 terior, or lining an internal cavity of the body, of fine pro- 

 cesses, which are capable of movement ; these processes, 

 which vibrate during the life of the cell, are known as 

 cilia. The hairs on these ciliated cells are sometimes 

 in the form of a single flag e Hum, or occur in a group 

 of many as cilia. In the former case the cell runs out 

 into a fine process, and forms a flagellate cell; these 

 are most common in the lower animals. Cilia are shown 

 to be differentiated products, since their movements are 

 not simply effected by the contractility which is already 

 inherent in the protoplasm. In many of the lower or- 

 ganisms cilia are formed for a time and are again drawn in and 

 their substance fused with the protoplasm. This shows that they 



Fig. 6. 



Flagellate 

 cells, a of 

 a Hydroid 

 Polyp, b of 

 a Sponge 

 (collar 

 cell). 



