No. 2.] VERTEBRATE CEFHALOGENESrS. 23 1 



the dorsal surface of the embryo, for a long time it lies open 

 and exposed to the sea-water ; its cells bear cilia which are 

 doubtless sensitive to various stimuli. Long before the neural 

 plate is converted into a cylinder (but only after the plate has 

 been overgrown by the lip of the blastopore), pigment makes its 

 appearance in some of the cells of the plate. At present we 

 have no complete account of the appearance or early relations 

 of this pigment matter ; but the mass called the eye-spot ap- 

 pears shortly after the first mass which arises in the middle of 

 the length of the plate, which at this stage falls in the fifth 

 somite. The eye-spot soon becomes much more important 

 than any of the posterior pigment-cell groups, and remains so 

 throughout life. As already stated, it at first faces upward, 

 but later acquires a facing at right angles to this direction on 

 account of the closure of the neuropore, the development of 

 the head fin, and the gradual thickening of the tissue directly 

 above the spot. Of course with this thickening of the body 

 walls, more and more light is cut off from the spot, and the 

 direction from which light reaches the spot in greatest abun- 

 dance is from the front and sides of the head. The head fin 

 fold divides the light falling upon the spot into two lateral bun- 

 dles of rays, which, owing to less thickness of tissue through 

 which they are compelled to pass, fall upon the sides of the 

 spot with greater energy than light from any other source. 



It is, then, owing to the relations of the external features 

 of the head that the primitively median pigment body (not 

 necessarily from an unpaired source) divides into two lateral 

 portions, each of which strives to get nearer the surface, and 

 in accomplishing the transposition cause an outgrowth of the 

 brain wall, producing the two antero-lateral diverticula spoken 

 of above. 



The relation of the two diverticula to the bases of the first 

 pair of nerves is of great interest in this connection, for, as we 

 know, the surface of the head end of the body of Amphioxus — 

 especially of the young — is pigmented along certain lines, over 

 certain areas ; and it appears highly probable that the pigment 

 cells of the surface have still some intimate relation to the light 

 and heat perception, — one or both. The pigment of the cen- 

 tral canal is derived from the pigment of the superficial ecto- 

 derm, which in the adult has nearly disappeared. 



