I02 AXAl'OMY OF AMFHIOXCS. 



1S76. who called it the lof'us olhic/orit/s. Kupffer has succeeded 

 in finding a similar structure m the embryos of other Vertebrates, 

 notably in Acipcuscr sti/rii? (xht sturgeon). He calls it the /ol'i/s 

 clfactorius impar, and shows that it indicates the point where the 

 medullary tube remained for the longest and last time in direct 

 connexion with the external ectoderm, precisely as is the case in 

 Amphioxus. There is thus at least one fixed point common to 

 the cerebral yesicle of Amphioxus and the brain of the craniate 

 ^'ertebrates. But Kupfter has found another. While it is obyious 

 that the anterior wall of the yesicle containing the pigment which 

 constitutes the eye-sf>ot is homologous with the primary optic tract 

 {recfssus opticus) of the higher Vertebrates, in which pigment is. 

 in many cases, deposited in the embryo. Kupifier states that he 

 is able to detect an infundibular depression m the floor of the 

 cerebral yesicle of Amphioxus. Immediately behind this depres- 

 sion there is a prominence in the wall of the yesicle. which Kupfter 

 calls the tuhin-tilum rosttriiis. This point is also to be identified 

 in the brains of the higher Vertebrates. 



The dorsal dilatation of the central canal, which Hatschek dis- 

 coyered and compared with the fourth yentricle of the yertebrate 

 brain, whose roof is similarly membranous and not ner\-ous (Fig. 

 45), is certainly a yery curious, and apparently constant, feature 

 in young indiyiduals, as I can affirm in confirmation of Hatschek. 

 The conclusion come to by Hatschek. howeyer. that the lobus 

 olfactorius of Langerhans is the homologue of the infundibulum of 

 the higher forms, would appear to be untenable in the hght of 

 Kupft'er's researches. 



It is beyond the scope of this book to discuss the difficult 

 problem of the origin of the paired eyes of the Vertebrates, but it 

 may be pointed out that there is no difficulty in identifnng a 

 stage in the embryonic deyelopment of the optic tract in the 

 Craniota corresponding to the permanent condition of things 

 in Amphio.xus. This fact was first demonstrated by Wilhelm 

 MuLLER in 1S74. On account of its position in front of and 

 below the cerebral yesicle, it is doubtful whether the eye-spot of 

 Amphioxus is homologous with the eye of the Ascidian tadpole. 

 (See below.") 



to. (p. 94.) It is a significant fact that giant nerie-fibres appear 



