THE NERVOUS SYSTEM OF AMPHIBIA 293 



established. Consequently the semicircular canals as such ai'e 

 not an essential factor in equilibration." He observed, further, 

 that tadpoles from which he removed one auditory vesicle in the 

 3 mm. stage began, when 9 to 9.5 mm. long, to differ from normal 

 tadpoles in the manner of swimming — the operated specimens 

 having a tendency "to swim with the operated side under, and 

 in the rolling movements around the long axis of the body it is 

 from the operated side under to the opposite." Streeter con- 

 cludes that it is at about this time that "functional union is 

 normally established between the ear vesicle and the spinal 

 cord reflex centers, upon which the individual is dependent for 

 maintaining its position in free water." Estimated by Street er's 

 notes, this is about two days after the animals begin to swim, 

 and, therefore, at a stage much beyond that which I call the early 

 swimming stage in Amblystoma. This difference in age is ex- 

 pressed in the structure of the ear as well as in the efficiency of 

 the motor system in locomotion, for the ear of the Amblystoma 

 early swimming stage is a perfectly simple sac, except for the 

 endolymphatic appendage — a much simpler organ than the ear 

 of the frog tadpole when it acquires the power to swim upright 

 in free water. Streeter notes that the auditory nerve is devel- 

 oped at this time, but does not describe its central relations in 

 detail. While it would be interesting to know these central 

 relations from the point of view of functional mechanism as a 

 whole, Streeter's results seem to be conclusive on the question of 

 the relation of the auditory organ as such to the function of 

 equilibration, and they do not lead us to expect any functional 

 union of the ear with the motor centers in embryos of Ambly- 

 stoma of the early swimming stage. 



3. The sensory nerve roots in relation to later development 



The medulla oblongata of Amblystoma of lengths from 17 to 

 38 mm. has been recently described by Herrick ('14). Such 

 larvae are practically adult so far as reactions are concerned and 

 in the structure of the medulla oblongata Herrick considers that 

 all the fundamental relations of the adult have been acquired. 



THE JOURNAL OF COMPARATIVE NEUROLOQY, VOL. 26, NO. 3 



