THE NERVOUS SYSTEM OF AMPHIBIA 163 



I. THE ANATOMICAL PART 



Embryos have been s'elected for this study according to the 

 physiological standards described in my earlier papers, namely: 

 (1) Embryos that move in response to an electric current but 

 not in response to tactile stimulation with a hair or similar non- 

 conducting and chemically inactive structure, designated as non- 

 motile stage; (2) Embryos taken very soon after there is percep- 

 tible movement in response to tactile stimulation, designated 

 early-flexure stage; (3) Embryos that move the trunk into a 

 coiled condition, designated the c oiled-reaction stage; (4) Em- 

 bryos that move to the right and left simultaneously in different 

 parts in a sinuous fashion, designated the S-reaction stage; 

 (5) Embryos that have just acquired the power of locomotion 

 through the serial S-reaction movements, designated the early- 

 swimming stage. 



For this study of the sensory system the S-reaction stage has 

 been omitted from the series. This omission has been made be- 

 cause the important differences between the S-reaction and the 

 early-swimming embryos relate to the commissural and motor 

 systems rather than to the sensory system, and do not offer any 

 distinctive contribution to the phase of the problem which this 

 paper treats. 



The histological methods employed in the preparation of the 

 material for study are noted in connection with the descriptions 

 of the several figures. 



1. THE GIANT GANGLION CELLS 



Since there are no perceptible dorsal roots from the funda- 

 ments of the spinal ganglia during the period under consideration, 

 the giant ganglion, or Rohon-Beard cells obviously constitute 

 the sensory system of the trunk at this time. It is with these 

 cells, therefore, that this paper preeminently deals. 



This series of neurones, which appears very early in the dorsal 

 portion of the spinal cord of aquatic vertebrates, has been the 

 subject of study on the part of anatomists from an early period 



