The embryological evidence shews that the ganglion-cells of the central part of the nervous system are originally derived from the simple undifferentiated epithelial cells of the surface of the body, while the central nervous system itself has arisen from the concentration of such cells in special tracts. In the Chordata at any rate the nerves arise as outgrowths of the central organ. Another important fact shewn by embryology is that the central nervous system, and percipient portions of the organs of special sense, especially of optic organs, are often formed from the same part of the primitive epidermis. Thus the retina of the Vertebrate eye is formed from the two lateral lobes of the primitive fore-brain. The same is true for the compound eyes of some Crustacea. The supraoesophageal ganglia of these animals are formed in the embryo from two thickened patches of the epiblast of the procephalic lobes. These thickened patches become gradually detached from the surface, remaining covered by a layer of epidermis. They then constitute the supraoesophageal ganglia; but they form not only the ganglia, but also the retinulæ of the eye--the parts in fact which correspond to the rods and cones in our own retina. The accessory parts of these organs of special sense, viz. the crystalline lens of the Vertebrate eye, and the corneal lenses and crystalline cones of the Crustacean eye, are independently formed from the epiblast after the separation of the part which becomes the central nervous system. In the Acraspedote Medusæ the rudimentary central nervous system has the form of isolated rings, composed of sense-cells prolonged into nervous fibres, surrounding the stalks of tentacle-like organs, at the ends of which are placed the sense-organs. This close connection between certain organs of special sense and ganglia is probably to be explained by supposing that the two sets of structures actually originated _pari passu_. We may picture the process as being somewhat as follows:--