THE XERVOUS SYSTEM OF AMPHIBIA 251 



verge towards the limiting membrane from among the yolk 

 globules that surround the nucleus. These yolk globules are 

 intracellular and certainly belong to the cell of this nucleus, so 

 that this strand of fibrils interspersed with yolk globules must 

 be a process of the cell. 



That these neurones with processes running out into the tri- 

 geminal nerve are not motor is clearly shown by reference to 

 figure 20, in which the motor nucleus of the nerve is easily estab- 

 lished in a much more ventral position (Xuc.Vis.m.). The cells 

 in question, therefore, must be giant ganglion cells, representing 

 an extension of the Rohon-Beard cells of the spinal cord into the 

 brain. As to the peripheral distribution of the fibers from these 

 cells, nothing can at present be said excepting that they appear 

 to enter both divisions of the trigeminal nerve. 



In this and in some of the later stages there appears just out- 

 side the brain and about the roots of the trigeminal nerve i)ar- 

 ticularly, but in a less degree about the roots of the other nerves, 

 a mass of cells which is quite distinct from the ganglia. It is 

 made up largely of indifferent cells, but cells resembling neuro- 

 blasts occasionally occur in it. This structure I have called the 

 root mass. It is indicated (R.M.) in figures 1, 2, 12, 15, 20 to 

 22, 34 to 37, 47 to 49, 09. There are no apparent grounds for 

 assigning to it any ])hysiological significance, and I have made 

 no effort to determine its origin, history or morphological sig- 

 nificance. 



B. THE EARLY FLEXURE STAGE 



By reference to figures 1 and 2 it will be seen that the oph- 

 thalmic ganglion has drawn somewhat nearer the brain. The 

 position of the ganglion with reference to the eye is further illus- 

 trated in figure 13 (G.oph.), and the attenuated nature of its 

 root at this stage is shown in figure 14 (R.oph.). The ganglion 

 has now broken away from its adhesion with the skin, but thick- 

 ened regions of the ectoderm indicate the areas of earlier attach- 

 ment {Ec.Th.). One of these ectodermal thickenings is shown 

 in figure 13. The two parts of the ophthalmic ganglion are still 

 perceptible. 



