20 NERVOUS SYSTEM AND SENSE-ORGANS 
connected with the brain and spinal cord, to which it is sub- 
ordinate, and its nerves branch out in the organs of digestion, 
circulation, &c. A few further details have already been given 
with regard to the nervous system of Man (see vol. i, p. 49). 
It was stated at the commencement of this section that the 
essential elements of the nervous system, z.¢. the neurons, are 
derived from the ectoderm or outer cell-layer. Considering 
that brain and spinal cord are far removed from the surface, 
while the body is traversed in all directions by nerves, it seems 
very difficult to believe 
such a statement, but the 
study of development 
shows that there is no 
doubt at all about the 
matter. 
At a comparatively 
early stage in the de- 
velopment of an embryo 
part of the ectoderm 
covering the upper sur- 
face thickens into a nerve- 
Fig. 1024.—Development of Central Nervous System in a plate, which sinks below 
Vertebrate Embryo, diagrammatic 
the surface, and at the 
a, Upper side of embryo, showing folding-up of the nerve-plate; . 
B, ¢ and D, stages in folding-up of nerve-plate, as seen in cross) Same time folds up to 
io ices  eOnstiuite the dere tube 
The details for the Lance- 
let have already been given (vol. iii, p. 345), but in that animal 
the nerve-plate sinks below the surface before it is completely 
folded into a tube, while in average cases the two processes go 
on simultaneously, as will be gathered from fig. 1024. 
The walls of the nerve-tube thicken, and by a process of 
unequal growth the spinal cord and the various regions of the 
brain come into existence. The rest of the nervous system 
grows out from the nerve-tube, e.g. the spinal nerves grow out 
from the spinal cord to the parts of the body which they supply. 
It therefore follows that these and the other nerves, as well as 
the sympathetic system, are really zxgrowths from the ectoderm 
or outer cell-layer, although in the adult they are far removed 
from the surface. 
Tue Brarn OF VERTEBRATES.—At first sight the brains of Fishes, 
Front 
