234 HOWARD AVERS. [Vol. IV. 



previously existing cavity, — every stage of whose phylogeny lies 

 before us in existing vertebrates. 



The chief merits o^ the theory I have here attempted to 

 establish is, that no demand is made upon the organism to sup- 

 ply any new structures, but simply to develop organs and struct- 

 ures already existing, in connection with a function of which we 

 have evidence enough that all animals endeavor to acquire in 

 ever increasing perfection, — i.e. light-perception and the possi- 

 bilities which grow out of it. 



F. The pigment of the eye-spot of Amphioxus is contained in 

 cells zvkich normally lie ijiside the bounds of the nerve mass, and 

 whenever fotmd outside in microscopical prrparatio7is, it is to be 

 considered as misplaced by chemical or mechanical means. 



Sections through the Amphioxus brain, prepared without con- 

 traction, or tearing, from well-preserved material, never show, so 

 far as my experience goes, the free pigment granules lying in 

 the interspace between the anterior end of brain and its sheath. 

 This point is worthy of mention, since the presence of such 

 free pigment particles has given rise to erroneous ideas as to 

 the structure of the eye-spot, and the nature of the pigment 

 deposit. 



G. The pigmejit bodies of the central nervous system of Am- 

 phioxus are connected with, a?tdform a part of segmental sensory 

 structures. 



These structures are doubtless derived from the superficial 

 sense organs of that ancestral form in which the neural plate 

 had not yet been converted into a tube, or had not received a 

 protective covering. 



As they exist in Amphioxus to-day, the regularity of their 

 disposition is somewhat interfered with by the rearrangement 

 of the cellular elements forming the bases of these structures, 

 which are now transformed into a part of the central nervous 

 apparatus. They consequently retain only indirect connection 

 with the periphery. 



After the formation of the neural plate as an embryonic or- 

 gan, the cells retain their embryonic condition for a varying 

 length of time. The transformation of these cells into the 

 elongated and greatly enlarged spherical, functionally active 



