264 G. E. COGHILL 



D. THE EARLY SWIMMING STAGE 



The facial and auditory ganglia have in this stage become still 

 more consolidated about the auditory vesicle and the roots enter 

 the brain farther caudad relative to the vesicle. 



1. The lateralline component. The preauditory lateral line 

 primordia have advanced greatly in distribution and differentia- 

 tion (Paper I, fig. 59). The supraorbital primordium can now 

 be traced far down in front of the eye and the olfactory organ 

 and spreads out at the end, in anticipation apparently of the 

 line of organs that develops later around the rim of the snout. 

 The suborbital primordium extends beneath the eye into close 

 relation with the olfactory organ, while the proximal portion of 

 this primordium is now continuous with the primordium of the 

 hyomandibular region. The latter primordium now passes ven- 

 trad behind the balancer, then mesad, where it appears on the 

 ventral surface behind the oral plate in two distinct ridges, in 

 which the characteristic organ structure .is well pronounced. 



Silver impregnations of the lateral line fibers now show them 

 extending practically the full length of the primordia, and follow- 

 ing rigidly the course of the primordia as in the earlier stages. 

 The nerve trunks are everywhere closely adhering to the epi- 

 thelium and here and there fine fibrils can be followed in among 

 the cells. 



The root fibers of the lateral line ganglia impregnate much 

 more deeply with the silver methods than do the peripheral 

 fibers. They enter the brain as illustrated in figures 55 to 60, 

 in two roots, R.L.L.VII, a and b). From a study of silver im- 

 pregnations of these fibers it is impossible to be absolutely sure 

 that all fibers from ganglion a enter root a, or that root b is 

 exclusively made up of fibers from ganglion b. But if there is 

 an intermingling of fibers from the different ganglia in the indi- 

 vidual roots this must be very slight. The roots, upon entering 

 the brain, form ascending and descending tracts, as shown in figure 

 6. The descending tracts reach the level of the middle of the 

 auditory vesicle, and the ascending tracts are of about the same 

 length or a little longer. The relation of the descending tracts 



