THE ORIGIN OF NERVES. 



293 



the development of the chick the sides of the groove (d d) 

 being seen gradually to close, and so to form the cerebro- 

 spinal canal ; whilst the first traces of vertebrae (e e) are also 

 observed. In D also, the first appear- 

 ance of the brain is also shown. 



The further development of the 

 chick is shown in Fig. 56, where the pre- 

 ceding stages are seen to have produced 

 the three dilatations of the brain (fb, 

 mb, hb\ and the beginnings of the eye 

 and ear are shown at op v and an p 

 respectively. The vertebrae, or joints 

 of the spine (pv) have become well 

 developed, and the. remains of the 

 'primitive groove" are shown at/r. 



Such is a brief sketch of the pro- 



L , ,, i i-r FIG. 56. Further develop- 



cesses which occur in the early life- ment of chick, 



history of every vertebrate animal, 

 man included. Let us now glance for a moment at the 

 part which each of the three layers of the young animal 

 plays in the formation of the various systems of the body ; 

 since thereby we may understand how the nervous system 

 is formed. From the "hypoblast" or undermost layer 

 (Fig. 54, H) the general lining membrane of the internal 

 parts of the body, such as the digestive system, is developed. 

 The middle layer or "mesoblast" (M) gives origin to the 

 tissues and organs of the body generally, except the brain 

 and spinal cord and the outer skin of the body, which are 

 formed from one and the same layer the " epiblast " (E). 

 Thus we arrive at the startling fact that the great nervous 

 centres of man and the higher animals are formed from 

 the same layer of the young being ivhich gives origin to 

 the skin or outer layer of the body. In other words, our 

 nervous centres are formed from an infolded portion of 

 what in the early condition was the outer layer of our 

 frame. This infolded part ultimately obtains, through the 

 development of connecting nerves, a communication with 



