STRUCTURE AND PHYSIOLOGY OF ANIMALS 



25 



a special part to play in the history of the development. As the 

 various parts of the embryo become gradually moulded from the 

 cells of the germinal layers, it becomes evident, on comparison, 

 that their internal structure the form and arrangement of their 

 constituent cells is undergoing gradual modifications, the nature 

 of which is different in the case of different parts. A differentia- 

 tion of the cells is going 

 on in the developing 

 organs, resulting in the 

 formation of a variety of 

 different kinds of tissues. 



4. TISSUES. 



The cells of the tissues 

 of the animal body differ 

 greatly in form in different 

 cases. Some are rounded, 

 others cubical, others 

 polygonal ; some are 

 shaped like a pyramid, 

 others like a cone, others 

 like a column or cylinder ; 

 others are flattened and 

 tabular or scale - like. 

 Some are Amoeba-like or 

 amoeboid, resembling 

 Amosba in their capacity 

 for developing pseudo- 

 pods. Cells situated on 

 free surfaces are in many 

 cases beset at their free 

 ends with delicate, hair- 

 like structures or cilia 

 which vibrate to and fro 

 incessantly during the life 

 of the ceU (Fig. 11, a) ; 

 sometimes there is on each 

 cell a single, relatively 

 long, whip-like cilium, 

 which is then termed a 

 fiagellum (f, g). Cells 

 provided with cilia are 

 termed ciliated, such as 

 bear flagella flagellate cells. 



Some tissues are composed entirely of cells. Usually the com- 

 ponent cells of a tissue are distinct ; but there are many examples 



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1*" I 



FIG. 11. Various forms of epithelium, a, ciliated epi- 

 thelium ; b, columnar ; d, surface view of the same ; 

 r, tesselated ; e, the same from the surface ; /, llagel- 

 late epithelium with collars ; g, flagellate epithelium 

 without collars ; Ji, epithelium of intestine with 

 pseudopods ; i, stratified epithelium ; k, deric epi- 

 thelium of a marine planarian with pigment cells, 

 rod-cells, and sub-epithelial glands. (From Lang's 

 Comparative Anatomy.) 



