356 ORR. [Vol. I. 



feature — the anterior union of the paired trunks — remains. 

 In both cases this anterior union of the paired trunks lies dorsal 

 to the mouth, and in the vertebrate embryo it is also dorsal to 

 the attachment of the notochord to the epiblastic hypophysis. 



I have drawn the above comparison because it seemed to me 

 to suggest a possible explanation of the primary condition of 

 the nerve-fibres in the nervous systems of Amphibia and Reptilia, 

 — in other groups observations on this point seem to be lack- 

 ing, — a condition that appears later either to degenerate or to 

 become so modified that it loses its primitive significance. The 

 continuous band of transverse ventral fibres which I have de- 

 scribed seems to be a secondary result of the close approxima- 

 tion of the lateral parts of the nervous system, and apparently 

 is not derived from the median epiblast connecting the two parts. 

 There are a number of minor points which seem -to accord with 

 this explanation, but it is unnecessary to enter into perhaps use- 

 less detail until the main features shall have been more widely 

 and strictly tested. 



Independent of any theory as to their origin, we have the fact 

 of the primitive lateral-longitudinal and anterior band of nerve- 

 fibres entering into peculiar relations with the optic organs. The 

 condition which I have described suggests an explanation. 

 The idea that the posterior or median wall of the optic vesicle 

 may once have performed the function of sight has already been 

 mentioned. Supposing this to have been the case, we would 

 have, then, in each of the paired optic vesicles an organ ho- 

 mologous with the pineal eye of Lizards' and Petromyzon ;'^ 

 that is, there would be in each case a continuous part of the 

 brain-wall lying between the true optic surface and the source 

 of light. As the paired vesicles arise primarily within the re- 

 gion of the primitive longitudinal fibres, these fibres coming 

 from behind would probably have run completely around the 

 longitudinal periphery of the vesicle, and continued their course 

 anteriorly to the lateral band of the opposite side. In this way 

 the posterior median wall of the vesicle would be covered with 

 fibres on its median surface, and, as is usual in simpler eyes, the 

 cellular elements would lie between the nerve-fibres and the 



' spencer. — Pineal Eye in Lacertilia. — Quart, your, of lilic. Sci., Vol. XXVII., 

 N.S. 



^ Beard. — The Parietal Eye in Fishes. — Nature, July 14, '87, No. 927., Vol. 36. 



