332 F. L. LANDACRE 



THE ORIGIN OF THE LATERALIS VII GANGLIA 



The origin of the lateralis Vllth ganglion resembles closely the 

 origin of the Gasserian with the exception that the lateral mass 

 (figs. 4 and 9, Stage III) giving rise to the lateralis Vllth never in 

 my series breaks down so completely into a loose mass of cells as 

 does the Gasserian. There is no break in continuity between the 

 lateral mass cells and the ganglion. Posterior to the hyoid gill 

 cleft, for some distance, the greater portion of the lateral mass 

 breaks down into mesectoderm, between Stages III and V, and 

 it is difficult to assign any definite boundary to the anterior end 

 of the lateralis Vllth ganglion at first. Its anterior end, as in 

 the case of the Gasserian, is situated more ventrally than the pos- 

 terior end and comes into contact with the mesoderm of the mandi- 

 bular arch anterior to the middle region of the arch. The anterior 

 portion of this lateral mass later forms part of the geniculate gang- 

 lion of the Vllth nerve, but at this time the anterior limit cannot 

 be outlined, not, in fact, until the placodal portion of the Vllth 

 is detached from the epidermis, which is some hours later (A. 

 nebulosus, Stage VIII). The whole ganglionic mass has an up- 

 ward and backward trend, finally coming into contact with the 

 auditory vesicle on its anterior, mesial and ventral walls. The 

 whole ganglionic mass of the lateralis Vllth is quite homogeneous 

 in structure, and shows no evidence of separating into the two 

 ganglionic masses, the dorso-lateral and ventro-mesial, of which 

 it is later composed. Its anterior boundary when it can be deter- 

 mined is not overlapped by the Gasserian ganglion but is over- 

 lapped by its root. Its posterior end, just where it comes into con- 

 tact with the vesicle, is more closely applied to the neural tube 

 than to the epidermis. The fibrillated root appears later at the 

 posterior end, where it is in contact with the middle region of the 

 cord. 



Fig. 11, from the same embryo as that from which fig. 10 was 

 taken, lies four sections anterior to the auditory vesicle. The 

 lateralis Vllth ganglion shows the same irregular boundaries 

 that characterized the Gasserian ganglion and is surrounded on 

 all sides, except that next to the epidermis, by loose mesectoderm 



