AND PHYSIOLOGY OF THE BRITISH ASSOCIATION. 707 



From embryology we learn that the ganglion-cells _of jthe 

 central part of the nervous system are originally derived from 

 the simple undifferentiated epithelial cells of the surface of the 

 body. We further learn that the nerves are out-growths of the 

 central nervous system. It was supposed till quite recently 

 that the nerves in Vertebrates were derived from parts of the 

 middle germinal layer or mesoblast, and that they only became 

 secondarily connected with the central nervous system. This is 

 now known not to be the case, but the nerves are formed as 

 processes growing out from the central part of the nervous 

 system. 



Another important fact shewn by embryology is that the 

 central nervous system, and percipient portion of the organs 

 of special sense, are often formed from the . same part of the 

 primitive epidermis. Thus, in ourselves and in other vertebrate 

 animals the sensitive part of the eye, known as the retina, is 

 formed from two lateral lobes of the front part of the primitive 

 brain. The crystalline lens and cornea of the eye are, however, 

 subsequently formed from the skin. 



The same is true for the peculiar compound eyes of crabs 

 or Crustacea. The most important part of the central nervous 

 system of these animals is the supra-cesophageal ganglia, often 

 known as the brain, and these are formed in the embryo from 

 two thickened patches of the skin at the front end of the body. 

 These thickened patches become gradually detached from the 

 surface, remaining covered over by a layer of skin. They then 

 constitute the supra-oesophageal ganglia ; but they form not only 

 the ganglia, but also the rhabdons or retinal elements of the 

 eye the parts in fact which correspond to the rods and cones 

 in our own retina. The layer of epidermis or skin which lies im- 

 mediately above the supra-cesophageal ganglia becomes gradually 

 converted into the refractive media of the crustacean eye. A 

 cuticle which lies on its surface forms the peculiar facets on the 

 surface of the eye, which are known as the corneal lenses, while 

 the cells of the epidermis give rise to lens-like bodies known as 

 the crystalline cones. 



It would be easy to quote further instances of the same kind, 

 but I trust that the two which I have given will be sufficient to 

 shew the kind of relation which often exists between the organs 



