2r)0 G. E. COCillll-i. 



not as extensive or as intimate as the adhesion between the 

 ()l)hthahiiic ganglion and the skin. In a transverse section of 

 the embryo this adhesion is seen in figure 9 to be ventral of the 

 ])rimordium of the preauditory lateral line organs. 



In the first section (10 m) ventrally of that of figure 12 the root 

 of the Gasserian ganglion enters the brain, while in the next 

 following section (20 ij. ) the root of the ophthalmic ganglion 

 enters. The axones of neither of these roots have grown beyond 

 10 or 20 M from the immediate zone of entrance. Throughout 

 their extent, however, they form a dense fibrillar mass imme- 

 diately beneath the external limiting membrane of the brain. 

 There is no clear evidence of bifurcation of the root fibers at this 

 stage. The axones which compose the root of the Gasserian 

 ganglion are illustrated in figure 19. The cells figured here occur 

 a considerable distance out from the brain and the longer processes 

 are the axones. The axones of the two cells of the figure come 

 into such close relation with each other that they might be inter- 

 preted as fusing together into a single root fiber, but they prob- 

 ably do not anastomose. The perikarya of these neurones or^ 

 neuroblasts, like those of all other ganglia of this age, are filled 

 with yolk spherules. 



As the roots of the ophthalmic and Gasserian gangha approach 

 the brain a small strand of fibers enters them from a more dorsal 

 position in the brain. The neurones from which these fibers arise 

 lie just inside the external limiting membrane and, mostly at 

 least, in a more dorsal position than the root entrance. They 

 have large spherical nuclei and the other morphological charac- 

 teristics of the Rohon-Beard cells of the spinal cord. In figure 

 20 (DC) one of the these cells is shown in a transverse section 

 of the medulla oblongata, and 15 ^ caudad of this appears another 

 such cell, which is reproduced in figure 21. The former neurone 

 sends its process to the immediate region of the entrance of the 

 trigeminal root while the latter (fig. 21) sends its process through 

 the external limiting membrane into the root of the nerve. 

 There are in this section two or three other neurones of this 

 type. At this stage the cell boundaries are rather indistinct, but 

 the fibrils which compose the emerging fibers of figure 20 con- 



