519 



especially the illustrations in Burckhardt's, Graham Kerr's and 

 Bing's memoirs). 



In have dealt with these various elements of confusion at the 

 outset to explain why I shall be compelled to set aside Burckhardt's 

 interpretation of the morphology of the Dipnoan hemisphere and his 

 terminology. I must, also, most emphatically protest agaiost the view 

 put forward by Burckhardt, and recently repeated and emphasized 

 by Bing, that the tuberculum olfactorium (Bing's "Lobus postolfactorius") 

 represents the true cerebral cortex, or, as Bing puts it, the "pros- 

 encephalon sensu strictiori" (p. 539). By no process short of calling 

 the dorsal surface of Lepidosiren "ventral" is such an interpretation 

 justifiable. 



Up till the time when the nasal sac, i. e. the olfactory organ, 

 is formed, the surface of the rudimentary cerebral hemisphere of 

 Lepidosiren, like that of other Vertebrata is in contact with the ex- 

 ternal ectoderm. But as the nasal sac is forming (in the manner de- 

 scribed by Graham Kerr, op. cit., p. 439) the forebrain becomes separ- 

 ated from the skin by a mass of mesoderm and is gradually pushed 

 away from the surface of the head. There is one area which is exempt 

 from this separation : the roof of the nasal sac, i. e. the olfactory 

 organ sensu stricto, remains in close contact with the lateral wall of 

 the cerebral hemisphere for some time after the rest of the latter has 

 become separated from the skin. Not only is the nasal sac in contact 

 with the cerebral hemisphere at this time (see Figures 4, 5 and 6), 

 but there is organic continuity between the two structures. A proto- 

 plasmic bridge extends from the nasal sac to the cerebral hemisphere 

 (Fig. 7), and in the neighbourhood of this connecting band I have been 

 unable to define the boundaries of the cells either in the brain or the 

 olfactory organ : in other words, the protoplasm of the one is in uninter- 

 rupted continuity with the other. As Holm^) long ago pointed out, when 

 describing the developmental history of the olfactory apparatus in Torpedo 

 ocellata, "a kind of connection always exists" (p. 204) between the 

 olfactory organ and the brain. In his excellent Fig. 2 Holm demon- 

 strates the earliest stage of this connection. 



It is important to note that the phase of development of the 

 olfactory connection, which we still find in the embryo of Lepidosiren 

 at Graham Kerr's stage 30 (Figures 4—7), is analogous to that found 

 by Graham Kerr in the case of the motor nerves at a somewhat 



1) Some Notes on the early Development of the Olfactory Organ 

 of Torpedo. Anatomischer Anzeiger, Bd. 10, 1895. 



