No. 2.] DEVELOPMENT OF PETROMYZON: 28 1 



trunk passes downwards closely applied to the brain-wall, and, 

 entering the ear-capsule, passes into the large and conspicuous 

 ganglion. At this stage I cannot detect the two auditory roots 

 so clearly shown in the adult, nor the division of the auditory 

 nucleus in the hind-brain. On the contrary, this nucleus ap- 

 pears to be continuous with the nucleus of the facial, and, 

 perhaps, even with that of the trigeminus. Only a very imper- 

 fect division of this elongated mass of cells is indicated. 



9-12. The Vagus Group. — The large complex of nerves 

 which arise immediately behind the auditory capsule has a 

 somewhat curious history. The earliest stage is shown in Fig. 

 36, PI. XI, where is seen a series of nerve-roots arising from 

 the hind-brain, each one provided with a ganglion, and all 

 connected by means of a longitudinal commissure. Owing 

 to the fact that the hind-brain, even in the earliest stages, 

 is overlapped by the mesoblastic somites, the first of which is 

 placed immediately behind the auditory vesicle, instead of 

 being separated from it by a considerable interval, as in the 

 Selachians, it is exceedingly difficult to say how many of these 

 roots belong to the cranial and how many to the spinal series. 

 The important fact to notice is that these vagus elements are 

 exactly like the dorsal roots of the spinal nerves, and form a 

 continuous series with those roots with which the commissure 

 connects them (Fig. 37, PI. XI). On account of this fact, and 

 the position of the mesoblastic somites, one cannot well deter- 

 mine the limits of the vagus group. This stage is essentially 

 the same as Balfour has figured in the Selachians; but the two 

 series are more closely continuous, and not separated by a 

 marked interval, as in those fishes. 



In a very short time {i.e., in embryos nearly ready for hatch- 

 ing, and in just hatched larvae) the appearance of the vagus 

 group has essentially changed, and a great concentration has 

 taken place (Fig. 3, PI. VIII, and Fig. 40, PI. XI). The gan- 

 glion of the ninth nerve is placed most inferiorly, and some- 

 what above and behind this are two ganglia, both of which 

 probably belong to the vagus. The ninth nerve has two 

 branches, one of which passes forwards over the hyobranchial 

 (first persistent) cleft and then downwards in front of it, and 

 the other straight downwards between the first and second 

 (persistent) clefts (2 and 3, Fig. 3, PL VIII). In the adult the 



