450 J. GRAHAM KERI?. 



paired structures, it seems probable that in tlie paired nature 

 of tlie velum we have also to do witli a primitive character. 



It is also seen from tlie study of sagittal sections tlirougli 

 the brain at succeeding stages of development that the 

 unpaired portion of the brain lying anterior to the level at 

 which the velum will later appear is nothing more than the 

 anterior portion of the thalamencephalon. It follows from 

 this that the paraphysis does not belong to the secondary 

 fore-brain at all ;^ it is a projection of the anterior wall of 

 the original thalamencephalon. 



Regarding speculations as to the original nature of the 

 paraphysis, Lepidosiren seems to offer little definite evi- 

 dence. On the whole, however, its topographical relations in 

 Urodeles and Dipnoans suggest that it is a part of the brain 

 wall which has been drawn out by the interposition of 

 mesenchyme between skin and brain, through its outer end 

 being attached to the skin ; in other words, that it probably 

 represents either an ancient sense-organ (Selenka) or a part 

 of the primitive connection of the brain with the outer skin. 



There has been considerable discussion as to the nature of 

 the smaller, more ventrally situated of the two vesicles 

 found in the pineal region of Petromyzon. The relations 

 of the paraphysis of Lepidosiren, about whose homology 

 there can be, I imagine, no doubt, support the view ex- 

 pressed by Kuplfer ^ that the vesicle referred to is also 

 morphologically the paraphysis. If this homology hold, the 

 discovery by Eetzius of nerve-fibres in the structure in 

 question in Petromyzon would, as Gaupp points out, sup- 

 port the idea of the at one time sensory nature of the para- 

 physis. 



The Peripheral Nerves. 



It will, I think, be admitted that the facts which I have 

 shortly sketched in regard to the development of the motor 



' Giauj)]), op. cit., p. 229. 

 » Op. cit., llcfl 2, S. 10. 



