HOMOLOGIES OF THE VERTEBRATE NERVOUS SYSTEM. 39! 



parts of the nervous system, and only thins out at a later 

 period. Further than this, any explanation of the thin roof of 

 the fourth ventricle ought also to elucidate the nearly similar 

 structure in the sinus rhomboidalis, and cannot be considered 

 satisfactory unless it does so. 



The peculiarities of the cerebro-spinal canal in the region 

 of the brain appear to me to present considerable difficulties in 

 the way of comparing the central nervous system of Vertebrates 

 and segmented Annelids. The manner in which the cerebro- 

 spinal canal is prolonged into the optic vesicles, the cerebral and 

 the optic lobes is certainly opposed both to an intelligible expla- 

 nation of the spinal canal itself, and also to a comparison of the 

 two nervous systems under consideration. 



Its continuation into the cerebral hemispheres and into the 

 optic lobes (mid-brain) may perhaps be looked upon as due to 

 peculiar secondary growths of those two ganglia, but it is very 

 difficult to understand its continuation into the optic vesicles. 



If it be granted that the spinal canal has arisen from a 

 folding in of the external skin, then the present inner surface of 

 the optic vesicle must also have been its original outer surface, 

 and it follows as a necessary consequence that the present 

 position of the rods and cones behind and not in front of the 

 nervous structures of the retina was not the primitive one. The 

 rods and cones arise, as is well known, from the inner surface of 

 the outer portion of the optic vesicle, and must, according to 

 the above view, be supposed originally to have been situated 

 on the external surface, and have only come to occupy their 

 present position during the folding in, which resulted in the 

 spinal canal. On a priori grounds we should certainly expect 

 the rods and cones to have resulted from the differentiation of 

 a layer of cells external to the conducting nervous structures. 

 The position of the rods and cones posterior to these suggests 

 therefore that some peculiar infolding has occurred, and may be 

 used as an argument to prove that the medullary groove is no 

 mere embryonic structure, but the embryonic repetition of an 

 ancestral change. The supposition of such a change of position 

 in the rods and cones necessarily implies that the folding in 

 to form the spinal canal must have been a very slow one. It 

 must have given time to the refracting media of the eye 



