94 PROF. MARSHALL AND MR. SPENCER. 



changes ; as shown in fig. 4, the dorsal attachment (fig. 1, v a) to the 

 top of the thickened side of the brain has disappeared completely, and 

 the nerve is now attached to the brain at a point about half way down 

 the side (fig. 4, v j3), i.e. at a point exactly corresponding to the 

 conspicuous projection (fig. 1, v /3) already described at the earlier stage. 

 Immediately beyond the root of origin the nerve enlarges, suddenly, 

 and presents a distinct dorsal projection at the base of the secondary 

 root of attachment. Although hitherto we have not succeeded in 

 following all the intermediate stages, and have not yet obtained 

 satisfactory preparations of the stages earlier than that drawn in fig. 1, 

 yet we feel justified in putting forward the following explanation of 

 the appearances we have just described, relying for our justification 

 partly upon the description given by Balfour, and still more on our 

 own observations on the development of the roots of the seventh nerve, 

 which will be described immediately. 



Balfour has described and figured the fifth nerve as arising at "stage 

 G, near the anterior end of the hind-brain, as an outgrowth from the 

 extreme dorsal summit of the brain, in identically the same way as 

 the dorsal root of a spinal nerve." 1 



He has further described 2 how, by the growth of the roof of the 

 brain, the nerves of the two sides, which at first are in contact dorsally, 

 shift their position and become widely separate. His descriptions and 

 our own somewhat fragmentary observations on these stages, when 

 considered in connection with our much more complete series of 

 observations on the seventh nerve, leave no room for doubt that the 

 root of origin (v a) shown in fig. 1 is the primary root, the one which 

 at stage a was situated at the top of the brain, and which has acquired 

 its present position merely in consequence of the rapid growth of the 

 roof of the brain pushing its two lateral halves apart, and so separating 

 the roots of the nerves. 



Concerning the root of attachment (V /3) shown at stage k in fig. 4 

 there is more room for dispute. Balfour appears to hold 8 that this 

 further change in position is due to the same cause as the former one, 

 i.e. to growth of the roof of the brain ; but this explanation, while it 

 would fully account for the first change, would in no way explain such 

 a shifting of the root down the thickened side of the brain, as is 



1 Op. cit., p. 191, and PI. XIV., fig. 3. 



2 Op. cit., p. 196. 



3 Op. cit., p. 196. 



