DEVELOPMENT 



821 



Fig. 315. Formation of the Neural 

 Canal at an Early Stage (Beard). 



origin are not situated in this part of the grey matter are not affected, 

 or at least completely recover. 



Certain tracts may also be marked out by means of the electrical 

 variation, which gives token of 

 the passage of nervous impulses 

 along them when portions of the 

 central nervous system or peri- 

 pheral nerves are stimulated 

 (Horsley and Gotch) . 



Development of the Central 

 Nervous System. Very early in 

 development (Fig. 315) the keel 

 of the vertebrate embryo is laid 

 down as a groove or gutter in the 

 ectoderm of the blastodermic area 

 (Chap. XIX.). The walls of this 

 ' medullary ' or ' neural ' groove 

 grow inwards, and at length there 

 is formed, by their coalescence, the ' neural canal ' (Fig. 316), which 

 expands at its anterior end to form four cerebral vesicles (Fig. 317). 

 Thus there is a continuous tunnel from 

 end to end of the primary cerebro-spinal 

 axis; and this persists as the central 

 canal of the spinal cord and the ven- 

 tricles of the brain, whose ciliated 

 epithelium represents the ectodermic 

 lining of the primitive neural canal. In 

 the adult portions of the canal may 

 become obliterated from an overgrowth 

 of the lining cells, and the cilia are, if 

 present at all, less distinct than in the 

 child, and far less distinct than in the 

 lower animals. From the wall of this 

 canal is formed the cerebro-spinal axis, 

 in which developing nerve-cells or neuro- 



Fig. 317. Diagram to illustrate 

 the Formation of the Cerebral 

 Vesicles. A. i indicates the 

 cavity of the secondary fore- 

 brain, which eventually becomes 

 the lateral ventricles. In B the 

 secondary fore-brain has grown 

 backwards so as to overlap the 

 other vesicles. I, first cerebral 

 vesicle (primary fore-brain or 

 'tween brain) ; II, second cerebral 

 vesicle (mid-brain); III, third 

 cerebral vesicle (hind-brain); IV, 

 fourth cerebral vesicle (after- 

 brain). 



Fig. 316. Neural Canal at a Later 

 Stage (Beard). C, neural canal; 

 G, posterior spinal ganglion. 



blasts soon become differentiated from the supporting cells or spongio- 

 blasts, and wander outwards from the neighbourhood of the central canal 

 (Fig. 328) till their further progress is checked by the barrier of the mar- 



