88 



MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



tive tissue intermingled with smooth muscle cells, blood-vessels, 

 nerves, etc., the whole being separated from the deeper tissues 

 by a layer of much looser connective tissue, usually containing 

 considerable fat. 



The structure of the adult epidermis varies considerably in 

 the different groups. In all it becomes several layers in thick- 

 ness, and is thicker in 



* 3 $ c f the terrestrial than in the 



aquatic forms. In the 

 ichthyopsida there is 

 slight differentiation be- 

 tween the layers, the 

 cells showing less strat- 

 ification than in the 

 higher groups. Among 

 them are numerous uni- 

 cellular glands, usually 

 spherical in shape, and 

 loaded with a slimy sub- 

 stance (mucus) ; and as 

 these cells approach the 

 surface they break, and 

 their contents spread 

 over the body, producing 

 the slimy condition so 

 familiar in these forms. 

 In the amniotes, on the 

 other hand, the outer 

 layers of the epidermis 

 undergo a hardening pro- 

 cess, and are converted 

 into a horny layer (stra- 

 tum corneum), Fig. 95, the beginnings of which are seen in the 

 frogs. Apparently the first layer to be budded from the basal 

 layer persists through a large part of the embryonic life as a 

 distinct sheet on the outside of the corneum, known as the 

 epitrichium, so called because in embryonic mammals it extends 

 in an unbroken sheet over the developing hairs. The non-corni- 



Co 



FIG. 93. Section of skin of lamprey eel 

 ( Petromyzon planert) from Wiedersheim. B, 

 mucous cells ; Co, derma ; CS, cuticular layer ; 

 Ep, epidermis ; F, fat ; G, blood-vessels ; Ko, 

 club cells ; Kb, granular cells ; S, IV, fibres of con- 

 nective tissue running vertically and horizontally. 



