338 ORR. ' [Vol. I. 



origin nearly similar to the ventral roots from the spinal cord. 

 Their point of exit will be described later. The first pair of 

 nerves spring laterally from the anterior dorsal (nasal) tip of the 

 primary fore-brain, and run a very short distance direct to the 

 nasal thickenings of the epiblast, in which they end. The nerves 

 of the third pair arise from the mid-brain ventrally, in the middle 

 region of the cranial flexure, and pass directly to the dorsal pos- 

 terior walls of the head-cavities. The fourth pair arises at a 

 time much later than the stages on which this work is based, and 

 is intimately related to the later histological differentiation of the 

 brain. The fifth nerve, arising as already noticed, passes for- 

 ward, dividing into a dorsal and a ventral branch. The latter 

 loses itself in the denser tissues which give rise to the structures 

 of the jaws. The dorsal branch (ramus opthalmicus) passes 

 forward dorsally to the head-cavity, and ends .close to the 

 dorsal surface of the eyeball ; later it gives off a branch to the 

 anterior wall of the head-cavity. The sixth nerve passes directly 

 from its ventral origin in the hind-brain to the ventral posterior 

 wall of the head-cavity. The seventh nerve passes down behind 

 the first gill-cleft. The eighth passes to the anterior wall of the 

 auditory vesicle. The ninth nerve passes down behind the 

 second gill-cleft. The tenth nerve passes to the median wall of 

 the first somite, which lies dorsally intermediate to the second 

 and third clefts. Shortly after its first appearance a dorsal 

 longitudinal commissure may be seen extending from this nerve 

 backwards through the region of about four somites. This 

 longitudinal commissure is irregularly attached to the brain-wall 

 by a number of small fibrous roots. Distally a few rudiments of 

 nerves may be seen extending from it a short distance. Two 

 ventral nerve-roots unite, at this later stage, with what I first 

 described as the tenth nerve. These nerves, together with the 

 commissure, represent the rudiment of the vagus. I have not 

 followed their development in detail. Behind the vagus-rudi- 

 ment all the nerves present the typical features of spinal nerves. 

 It is worthy of notice that between the neuromere of the fifth 

 nerve, which sends a branch down behind the mouth-opening, 

 and the neuromere of the seventh nerve, which passes down 

 behind the first gill-cleft, there is an intermediate neuromere. 

 Also between the neuromere of the seventh and the neuromere 

 of the ninth nerve, which passes down behind the second gill- 



