Dec, 1916] Prof itudus and Gasserian Ganglia in Plethodon 29 



The 9 mm. embryo (Fig. 30 and Fig. 1-6) is the oldest stage 

 of my series. It shows a fused condition of the profundus and 

 Gasserian gangha and at the root it is impossible to distinguish 

 between the two portions of the ganglionic complex. 



At the anterior end, anterior to the level of the optic stalks, 

 the profundus ganglion consists of a small mass of pigmented 

 cells dorso-mesial to the optic vesicle and in contact with it 

 (Fig. 2 and 3 Prof. G). At the level of the optic stalks the 

 profundus is a larger oval and compact mass (Fig. 3, Prof. G), 

 in about the same position as in the more anterior sections. 

 At a point about one-half of the total length of the ganglion 

 from the anterior end, the ganglion becomes larger and round in 

 cross section, and lies mesially towards the ventro-lateral 

 border of the medulla. Just posterior to the optic vesicle 

 the profundus and Gasserian ganglia are joined and attached 

 by a fibrous root to the medulla at its ventro-lateral border. 

 In these sections the first visceral pouch of the pharynx is 

 beginning to appear as an evagination of the endoderm (Fig. 5, 

 Ph. P. I) accompanied by condensation of mesenchyme. 



Posterior to the root of the ganglionic mass the Gasserian 

 portion forks over the first visceral pouch in about the position 

 where the maxillary V and mandibular V nerve trunks arise 

 in later stages (Fig. 6, Gass G.). These trunks are entirely 

 cellular at this stage. Near its posterior end the Gasserian 

 ganglion becomes loose in texture. The dorsal lateralis gang- 

 lion on VII appears dorso-lateral to the Gasserian in the most 

 posterior sections of the Gasserian. (Fig. 6, D. L. VII). 

 No histological differences between the two ganglia are apparent 

 at this stage (9 mm.) although in the 11.5 mm. stage the nuclei 

 of cells of the lateral line ganglia are larger and lighter staining. 



Description of the 7 mm. stage. 



At this stage the profundus ganglion is very large and more 

 prominent than the Gasserian. It is in contact with the Gas- 

 serian at its proximal end, but enters the medulla by a separate 

 root. The profundus projects anteriorly from its point of 

 contact with the Gasserian and lies dorsal to the optic vesicle. 

 The Gasserian consists of a mass in contact with the medulla 

 and of a ventro-lateral projection. The dorsal lateralis 

 ganglion on VII lies lateral to the distal part of this extension. 

 The general relations are shown in the flat reconstruction. 

 (Fig. 31). 



