18 



MORPHOLOGY OF THE 



The chief point of difference between this embryo and 

 Figure 8 consists in the entrance of the nerve fibres derived 

 from the Y ganglion into the medullary wall. It is here 

 that these fibres enter into the formation of the tractus 

 solitarius. The remaining fibres have increased very much 

 in size, through the growth of the embrj^o, — indeed, an 

 entirely new branch of this nerve is seen. 



Figure 14 represents a similar section made through 

 the (YII and VIII) ganglia of an embryo 14 mm. in length. 

 This section differs materially from that of the younger 

 embryo shown in Figure 9. The fibres derived from the 

 vestibular ganglion can be followed much deeper into the 

 medullary wall, to form the tractus solitarius, than in 

 the section of the younger embryo. 



This corresponds with the position of the tractus soli- 

 tarius, in the sections cut parallel to the floor of the medulla 

 in the 13.6 mm. embryo. Moreover, fibres from the ganglion 

 cochlearis (w^hich were entirely absent in Figm^e 9) cross 

 the root of the geniculate ganglion and unite with fibres 

 derived from the vestibular. 



This being the case, I am unable to state with certainty 

 whether fibres derived from the vestibular, cochlea, or genic- 

 ulate ganglia all enter into the formation of the tractus- 

 solitarius, or whether one set alone aids in its formation. 

 Should this last supposition be correct, judging from the 

 preparations it would seem to me most likely that they 

 were derived from the ganglion vestibularis. 



Transverse sections of the nerve roots of the IX and 

 X ganglia of this same embryo failed to show anything of 

 importance relating to our subject. 



On comparing these sections with each other, it seems 



