206 Proceedings of the Royal Fhysical Society. 



the phenomena observed in them — we have a further dis- 

 turbing factor in the differential growth seen in the brain 

 region. This is correlated with two facts — (1) that associated 

 with the high complexity of the adult brain, there occurs 

 precocious increase in the size of the brain, and (2) that 

 owing to the enormous mass of yolk, the ventral side of 

 the embryo is kept back in its increase in length, while the 

 dorsal side, going on with its development, causes the axis 

 of the brain to become greatly curved. 



Personally, I am inclined to conclude that the correct view 

 as to the primitive division of the vertebrate brain is that 

 which divides it into two (rhombencephalon and cerebrum), 

 a view now accepted by many anatomists on the grounds of 

 adult structure. 



The second point is in regard to the morphology of the 

 fore-brain region. The evidence of all three of the forms 

 under consideration agrees in supporting the old view — 

 supported in these days especially by Studnicka — that the 

 hemisphere region is primitively paired, and absolutely 

 against the more generally accepted view that the hemi- 

 spheres are to be looked on as the terminal region of the 

 brain, as a " telencephalon " in the sense of His. 



From the evidence of the more primitive vertebrates, 

 where the brain region is not distorted by the presence of 

 the yolk-sac hindering the growth in length of the ventral 

 side, it must, I think, be believed that the hemispheres are 

 to be regarded simply as localised developments of the 

 lateral walls of the primitive fore-brain, developments of a 

 similar kind to those which are developed in connection 

 with the special sense of sight, and form the optic "out- 

 growths." 



Motor Nerves. 



In my description of the development of the motor nerves,^ 

 the chief point brought out is that in Zepidosiren it is 

 possible to demonstrate the fact that the motor nerve which 

 passes from the central nervous system to the peripheral 

 muscles can be traced back to an extremely early stage of 



1 Trans. Roy. Soc. Edin., vol. xli. p. 119. 



