336 ORE. ' [Vol. I. 



space lateral to it is occupied by the auditory vesicle, so that 

 the fourth might be considered the original neuromere of the 

 auditory nerve. The fifth neuromere gives off, also dorsally, 

 the root of the ninth nerve. Immediately posterior to the fifth 

 neuromere is another similar swelling, which, however, is not 

 separated by so definite a ridge and depression from the pos- 

 terior neural tube into which it blends. This swelling gives 

 off, dorsally, the tenth nerve ; so we are justified in calling it a 

 neuromere. Anterior to the neuromere of the fifth pair of 

 nerves, the lateral brain-wall continues forward in a long curve 

 to the constriction marking the rear boundary of the mid-brain. 

 At this constriction there is a slight external depression and in- 

 ternal ridge, thus giving to the region of the long curve (in a very 

 young embryo) the appearance of a neuromere nearly three times 

 as long as the posterior neuromeres. There is no nerv^e for this 

 neuromere, unless we suppose that primitively some relation- 

 ship existed between it and the fourth nerve, which arises at a 

 very much later period in the region of the anterior limiting 

 constriction. I know of no evidence to support such a view. 

 At the time of the earliest development of the neuromeres 

 which I have described, the mid-brain appears to be simply an 

 enlarged neuromere. Anterior to the mid-brain in the region 

 of the thalamencephalon are two neuromeres, which are similar 

 to the characteristic neuromeres of the hind-brain, except that 

 they never give off any nerves. Fig. 6, PI. XII., shows these 

 neuromeres in a more advanced embryo ; anterior to them is 

 the swelling of the secondary fore-brain. As the nerve-fibres 

 in the brain begin to develop, the constrictions marking off 

 those neuromeres, posterior to the division of mid- and hind- 

 brain, gradually disappear. There are no neuromeres behind 

 the tenth nerve. As the embryo approaches the time of hatch- 

 ing, the cartilaginous dorsal arches of the vertebrse seem to 

 cause a regular constriction of the medulla; but at this time 

 the dorsal and ventral fissures have appeared, and there are no 

 ridges in the small central canal. This different appearance 

 and apparently different origin do not allow any very direct 

 homology of these parts of the medulla with the neuromeres 

 of the brain. Balfour described certain internal swellings of 

 the lateral wall of the hind-brain of elasmobranch embryos: 

 " Swellings of the brain towards the interior of the fourth ven- 



