294 G. E. COGHILL 



A comparison of his descriptions with the anatomical results of 

 this paper will show that the general plan of the sensory centers 

 of the medulla oblongata is laid down by the growth inward of 

 the root fibers of the peripheral nerves. The trigeminal and 

 lateral line fibers have formed extensive ascending and descend- 

 ing tracts, and the auditory and geniculate ganglia have formed 

 descending tracts of considerable length by the time the animal 

 begins to swim. These tracts are all closely grouped and all 

 lie immediately beneath the external limiting membranes, and 

 constitute the substantia alba of the dorsal portion of the medulla 

 oblongata. They lie embedded in the marginal zone of the sub- 

 stantia grisea, in which are neurones directed dorso-ventrad 

 across the whole series of tracts of the various sensory modes. 

 In the neurones of this group there can scarcely be any specificity 

 of function with reference to the different sensory nerves (com- 

 pare Herrick and Coghill '15). To this elementary plan the 

 adult has added the longitudinal association tracts a and h of 

 Kingsbury and Herrick, and an additional lateral line tract, 

 while the fasciculus solitarius has become detached from the 

 perip'hery and with its displacement inward the lateral line centers 

 have developed greatly ventrad as well as dorsad. 



Without knowledge of intermediate stages between the early 

 swimming stage and the younger of Herrick's descriptions it is 

 impossible to say just how the three root bundles of the lateral 

 line VII of Herrick are derived from the two bundles of the 

 younger stage. Since, however, the root bundles of the younger 

 stage have the same relation to each other and to the root bundles 

 of the postauditory system as do the two more ventral bundles 

 of Herrick, and since the most dorsal bundle of Herrick has, in 

 younger stages, smaller fibers than the other bundles, it is reason- 

 able to infer that the most dorsal bundle of Herrick is a distinct 

 and newly acquired tract, and not a detached portion of one or 

 the other, or of both of the tracts of the younger stage. 



In the relation of the several root bundles to particular ganglia 

 Herrick finds that, in the case of the facial division, each of the 

 tracts receives fibers from both the preauditory ganglia. This 

 is a very difficult relation to determine in the young embryos, 



