IO2 ANATOMY OF AMPHIOXUS. 



1876, who called it the lobus olfactorius. Kupffer has succeeded 

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

 notably in Acipenser sturio (the sturgeon). He calls it the lobus 

 olfactorius 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 vesicle of Amphioxus and the brain of the craniate 

 Vertebrates. But Kupffer has found another. While it is obvious 

 that the anterior wall of the vesicle containing the pigment which 

 constitutes the eye-spot is homologous with the primary optic tract 

 (recessus opticus} of the higher Vertebrates, in which pigment is, 

 in many cases, deposited in the embryo, Kupffer states that he 

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

 cerebral vesicle of Amphioxus. Immediately behind this depres- 

 sion there is a prominence in the wall of the vesicle, which Kupffer 

 calls the tubercuhtm posterius. This point is also to be identified 

 in the brains of the higher Vertebrates. 



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

 covered and compared with the fourth ventricle of the vertebrate 

 brain, whose roof is similarly membranous and not nervous (Fig. 

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

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

 The conclusion come to by Hatschek, however, that the lobus 

 olfactorius of Langerhans is the homologue of the infundibulum of 

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

 Kupffer'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 identifying a 

 stage in the embryonic development of the optic tract in the 

 Craniota corresponding to the permanent condition of things 

 in Amphioxus. This fact was first demonstrated by WILHELM 

 MULLER in 1874. On account of its position in front of and 

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

 Amphioxus is homologous with the eye of the Ascidian tadpole. 

 (See below.) 



10. (p. 94.) It is a significant fact that giant nerve-fibres appear 



