270 THE EVOLUTION OF MAN. 



reached if we see it clearly before us. I will now, there- 

 fore, mention as briefly as possible the relations which 

 these particular organs of the vertebrate organism bear to 

 the four different germ-layers. 



The first of the secondary germ -layers, the skin-sensory- 

 layer, produces, — firstly, the outer covering of the whole 

 body; the outer skin, or epidermis, and, in higher Ver- 

 tebrates, the hair, nails, sweat and sebaceous glands, and 

 all other parts developing secondarily from the originally 

 simple outer skin (epidermis). In the second place, from 

 this layer arises also the central nerve-system, the medullary 

 or spinal canal. It is remarkable that this mental organ 

 develops from the outer surface of the epidermis, and, only 

 afterwards, during the course of the development of the 

 individual, gradually moves inward, so that, at a later 

 period, it is situated internally, surrounded by muscles, 

 bones, and other parts. Thirdly, the primitive kidney of 

 the Vertebrate which secretes the urine, probably develops 

 from the outer germ-layer. It may be presumed that this 

 primitive kidney was originally a secretory gland of the 

 skin, like the sweat-glands, and, like them, developed from 

 the outer skin (epidermis); at a later period it lies deep 

 within the body. 



From the second of the secondary germ-layers, the skin- 

 fibrous layer, arises the principal mass of the vertebrate 

 body, namely, all those parts lying between the epidermis 

 and the inner coelom, and forming the firm body- wall. To 

 these belong, firstly, the leather-skin (corium), which lies 

 at the surface directly under the epidermis, — the firm, 

 fibrous covering which contains the nerves and blood-vessels 

 of the skin; secondly, the great masses of muscle of the 



