CHAPTER. XIV 
NERVOUS SYSTEM AND ORGANS OF SPECIAL SENSE 
THE nervous system consists of the brain and the spinal cord, 
and of the cranial and spinal nerves. The rudiment of the 
future brain and spinal cord first appears in the embryos of 
some Cyclostomes (e.g. Bdellostoma), of Elasmobranchs, and of 
Chondrostei (e.g. Acipenser), and of Neoceratodus among the Dipnoi, 
in the form of a tubular medullary canal pinched off from the 
epiblast of the dorsal surface of the body. sy a somewhat 
different method, but with the same final result, a medullary 
canal is formed in other Cyclostomes (e.g. Petromyzon), in the 
Holostei and Teleostei, and in JLepidosiren,' from a solid in- 
growing keel of epiblast which subsequently becomes tubular. 
Later, the medullary canal in the head enlarges, and becomes 
divided by two transverse constrictions into three vesicles, the 
primary fore-, mid-, and hind-brain, leaving the rest of the canal 
to form the spinal cord. 
The Spinal Cord.—This portion of the medullary canal 
retains a simpler and more uniform cylindrical structure. Its 
walls thicken and their component cells become converted into 
nerve cells and nerve fibres, but a remnant of the original 
cavity remains in the adult as a minute axial canal, with a 
ciliated epithelial lining, the central canal of the spinal cord or 
myelocoele. In most Fishes the spinal cord extends the whole 
length of the body, but in some Teleosts, especially in certain 
Plectognathi, it is remarkably short. In a Sun-Fish (Orthago- 
riscus), 24 metres long, and weighing about a ton and a half, the 
cord was only 15 mm. in length, or shorter than the brain. 
The Brain.—At an early stage in its embryonic history the 
1 Graham Kerr, Quart. Journ. Micr. Sci. xlvi. 1902, p. 417. 
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