28 The Ohio Journal of Science [Vol. XVII, No. 2, 



In the course of the study a count was made of the nuclei 

 in the ganglia at each stage. In making this count the principle 

 was adhered to of counting every nucleus visible in cross section. 

 Many nuclei were thus recounted, but the error is apparently 

 constant for all stages and would not effect the ratio in the 

 results. 



General Features. 



The profundus and Gasserian ganglia which together 

 form the trigeminus complex lie in front of the auditory vesicle 

 and are the most anterior of all the pre-auditory ganglia in 

 this form, except the ganglion of the nervus terminalis. Their 

 general position is anterior to the facial ganglia and the auditory 

 vesicle and in the region of the optic vesicle. 



The profundus may, however, extend as far anterior as 

 the region of the nasal capsule and the Gasserian as far posterior 

 as the first visceral pouch of the pharyngeal region. 



The ganglia are usually more or less completely surrounded 

 by the loose mesenchyme, which fills the head region. The 

 mesenchyme cells have rather large, light staining nuclei and 

 unpigmented cytoplasm containing very large yolk granules and 

 seems to be composed of mesentoderm. The cells of the ganglia 

 have the opposite characters of small, darkly staining nuclei, 

 like the ectoderm and small, obscure yolk granules. 



This contrast in histological character makes it possible 

 to determine the limits of the ganglia and the mesenchyme 

 at any stage. 



Description of the 9 mm. stage. 



In the 11.5 embryo of Plethedon gl. described by Kostir, the 

 Gasserian ganglion (Fig. 29, Gass. G.) is a large mass, oblong in 

 cross section lying wedged between the brain and the optic 

 vesicle. The anterior end of the profundus portion extends 

 for some distance forward between the brain and the optic 

 vesicle. From the ganglionic complex arise three nerve trunks: 

 (a) The opthalmicus V from the anterior end; (b) the maxillaris 

 V, and (c) the mandibularis V which fork over the temporahs 

 and masseter muscles. 



The trigeminus ganglion is attached to the medulla by a 

 fibrous root, long antero-posteriorly. The nuclei of the ganglia 

 have the usual dark staining properties and at this stage are 

 very closely packed together. 



