56 OUTLINES OF EVOLUTIONARY BIOLOGY 



epidermis, in which cell-division goes on actively throughout life. 

 In this way a many-layered or stratified epithelium is formed, as 

 shown in Fig. 18, which represents a small portion of a thin 

 vertical section through the epidermis (epd.) in the mouth of a 

 foatal cat. At the lower limit of the epithelium, resting imme- 

 diately upon the connective tissue of the dermis (der.), is seen 

 the layer of actively dividing cells (s.m.). The cells cut off from 

 this layer are gradually pushed outwards, becoming flattened and 

 scale-like as they approach the surface. 



Fat, or adipose tissue, consists of an aggregation of more or 



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FIG. 20. Section of Cartilage, showing the Cartilage Cells (c.c.) imbedded in 

 the transparent intercellular Matrix or Ground Substance (in.), X 390. 

 (From a photograph.) 



less globular cells, swollen out by the accumulation within them 

 of drops of oil. If the oil is dissolved out by suitable reagents 

 the empty cells are left with their cell- walls or membranes in a 

 somewhat shrivelled condition, as shown in Fig. 19, and the 

 tissue now bears a curious resemblance, when seen in section, to 

 vegetable parenchyma, such as is seen in sections of pith 

 (compare Fig. 7). 



Cartilage, or gristle, is one of the skeletal tissues, serving for 

 the support of the body and the protection of special organs. 

 In some of the lower vertebrates, such as the dog-fish, it forms 

 practically the whole of the internal skeleton, but in higher forms 



