2l6 THE PINEAL ORGAN 



developed from a median portion of the pigmented eye o£ Amphioxus. . . . 

 The rudiments of the eye were developed from (segmental) sense-organs, 

 but the eye itself is never developed from two right and left halves, in so 

 far as the closure of the medullary folds would necessitate this." 



We have quoted these extracts in full because they illustrate, besides 

 the important observation with regard to the bilateral disposition of the 

 pigment of the eye-spot in Amphioxus, several errors of interpretation 

 which were common at the time, more especially with regard to the position 

 of Amphioxus in the vertebrate phylum, namely — not as an offshoot from 

 a branch of the main stem, but as a living representative of the parent 

 stock from which the various higher types of vertebrates, including the 

 Cyclostomes, have descended ; secondly, the assumed visual function of the 

 pigment spot of Amphioxus which has been disproved (see pp. 178, 179) ; 

 thirdly, the denial of the possibility of two laterally situated sense-organs, 

 right and left, becoming united in a single organ : this we now know occurs 

 normally in the formation of the triple median eye of arthropods which is 

 produced by the fusion of the right and left members of one of two pairs 

 of ocelli, the other pair coming into contact but not being completely 

 fused ; fourthly, the failure to recognize the degenerate condition of the 

 anterior end of Amphioxus, the whole animal being regarded as having 

 reached only a very simple stage of evolution, whereas it is in many respects 

 highly evolved, and it is probable that the head-region was in the less 

 remote ancestors of Amphioxus much more highly developed than it is in 

 the living representatives of the species. It seems more probable that the 

 pigment-spot of Amphioxus instead of " representing the earliest stage in 

 the phylogenetic development of the vertebrate eye " is merely a vestige of 

 a more highly developed organ, or possibly a pair of light-percipient sense- 

 organs which were present in the head of the remote ancestors of the 

 present stock, before the latter took to burrowing head downwards in the 

 sand at the bottom of the sea. 



Polypterus Bichir l 



Another example of enormous development of the dorsal sac and special 

 modification of the pineal organ is found in the primitive Crossopterygian 

 ganoid fish Polypterus, found in the Congo and Upper Nile. In this 

 fish the dorsal sac arises by a relatively narrow stalk and expands as 

 it extends upwards towards the cranial vault. It thus appears triangular 

 in a sagittal median section, the apex of the V being directed downwards. 

 The posterior wall of the sac is folded by vascular ingrowths, but else- 

 where it is smooth and membranous. The pineal organ consists of a 



1 J. Waldschmidt, 1887, "Beitrag zur Anatomie des Zentralnervensystems und des 

 Geruchsorganes von Polypterus bichir," Anat. Anzeiger., Jahrg. 11. 



