THE NERVOUS SYSTEM OF AMPHIBIA 281 



from the territory of the vagus nerve; and that, as compared with 

 stimuU from these areas, stimuU very rarely reach the motor 

 centers from the trigeminal field in embryos of the tj^jical coiled 

 reaction stage. At the time these experiments upon this sub- 

 ject were made we had no exact information upon the nature of 

 the innervation of the regions under consideration. The ana- 

 tomical part of this paper, however, shows that the trigeminal 

 nerve, throughout the whole period of development up to the 

 early swimming stage, runs far ahead of the general cutaneous 

 component of the vagus in the development of both peripheral 

 and root fibers. The relative accessibihty of motor centers, 

 therefore, to stimuh from trigeminal and vagus territories can 

 not depend upon the relative development of these nerves. The 

 certainty of responses, when the stimuli are applied to the spinal 

 region, obviously depends upon the perfection of the giant gan- 

 glion cell system of afferent neurones and their direct access to 

 motor centers (Paper I). The neurones of this system occur as 

 far rostrad as the second myotome, which is essentially within the 

 limits of the medulla oblongata. They may, indeed, occur here 

 and there farther cephalad as shown in this paper. Their pe- 

 ripheral fibers, therefore, probably invade the region which was 

 regarded in the above mentioned experiments as belonging to 

 the vagus, but in a less efficient manner than in the trunk, so 

 that response is not as certain from stimulation here as it is from 

 stimulation farther caudad. The response to stimulation of the 

 whole postauditory territory and trunk is in all probability 

 effected through the giant ganghon cells system in the coiled- 

 reaction stage. And since we now recognize cells of this type 

 in the trigeminal nerve, such responses as occur to stimulation 

 of this region in the earlier periods may take place through this 

 system also. Granting, however, that all the giant ganglion 

 cells in the trigeminus are distributed to the skin rather than to 

 the muscle primordia, the innervation of the preauditory region 

 by this system must be very sparse as compared with that of 

 the postauditory and trunk regions, for there are only a few 

 giant ganglion cells in the vicinity of the trigeminal root. Such 

 sparseness of inner\'ation, upon the hypothesis that the gan- 



