92 - PROF. MARSHALL AND MR. SPENCER. 



brain ; it is, therefore, of great importance to notice that from the 

 very earliest stage at which we have seen it the point of origin is that 

 of the adult nerve. 1 Inasmuch as the majority of the cranial nerves, 

 as well as the dorsal roots of the spinal nerves, arise at first from the 

 dorsal surface of the brain or spinal cord, it seems natural to suggest 

 that the fourth nerve differs from all the rest, not in its mode of 

 origin, but in the fact that, whilst all the other nerves shift their 

 attachment to a greater or less extent, it alone preserves the primitive 

 position of its roots of origin. This shifting of the roots is, in part, 2 

 due to the rapid growth of the dorsal part of the brain forcing the 

 roots of the two sides from each other ; and it becomes of interest to 

 notice that the fourth nerve arises from a portion of the brain where 

 this rapid growth of the roof does not occur, and where, consecpaently, 

 one cause of the change in the other nerves is absent. 



The fact that the direction of the fourth nerve is at first at right 

 angles, or nearly so, to the axis of the part of the head from which it 

 arises is of importance, as showing that the fourth nerve comes under 

 the category of segmental nerves f and inasmuch as there is no room 

 as regards visceral arches and clefts for a segmental nerve between the 

 third and fifth the fourth would probably be rightly viewed as a 

 separated branch of the third — the only other nerve arising from the 

 mid-brain. 



On the other hand, certain other facts in connection with the fourth 

 nerve cannot be explained so easily. Thus, segmental nerves not only 

 arise from the neural crest ; they also arise early, are from the first 

 large, and have ganglia at or near their roots. Now, so far as our 

 observations go, the fourth nerve, though it may possibly arise from 

 the neural crest, would not appear to arise early, and certainly is not 

 at first a large nerve, being of much greater size at stage o-p (fig. 1 5) 

 than at stage n (fig. 11) ; whilst at stage l, in the single specimen in 

 which it has been detected, it is a nerve of extreme slenderness; 

 moreover, at no part of its length have ganglion .cells been found — an 

 important point of difference from segmental nerves. 



Another curious feature concerning the fourth nerve is that, in our 

 embryos, it appears almost constantly to divide close to its root into 

 two or even more branches, as is well shown in fig. 11, iv. These 



1 Balfour, ' Elasmobranch Fishes,' pp. 156 and 191. 



2 We say "in part," because it will be shown further on in this paper that another process 

 contributes greatly to this shifting. 



9 Marshall, "Morphology of Olf. Organ," p. 318, 'Quart. Journ, Micr. So.,' July, 1879< 



