190 G. E. COGHILL 



portion is sensory. Furthermore, that the sensory and motor 

 tracts of the same side are physiologically distinct and separate 

 structures throughout the greater part of their extent is proved by 

 the fact that the caudal piece of embryos that have been transected 

 at certain levels, varying with the age, have no power of response 

 to stimulation, either by tactile or chemical means. In fact, under 

 normal conditions, the sensory and motor tracts of the same side 

 seem to be absolutely isolated from one another physiologically, 

 for reaction to a stimulus on one side of the embryo is typically 

 followed by a contraction in the muscles of the opposite side, 

 as my earlier work on Diemyctylus showed and as my later ex- 

 periments on Ambly stoma confirm. The explanation for the 

 apparent exceptions to this rule in response is mentioned later 

 on in connection with the proprioceptive functions of the giant 

 ganglion cells. 



The correlation of anatomical and physiological evidence, 

 therefore, gives ample justification for interpreting the reflex 

 mechanism of the trunk of these embryos as made up of a dorsal 

 afferent system consisting of the giant ganglion cells, of a ventral 

 motor system composed of a continuous, conducting column of 

 neurones which innervate the muscles, and of an associative 

 system, which, in the rostral portion of the cord, connects the 

 afferent system of one side with the motor system of the other. 



2. THE RECEPTIVE FUNCTIONS OF THE GIANT GANGLION CELLS 



a. The interoceptive field 



There is no evidence that the giant ganglion cells have an in- 

 teroceptive field of stimulation or that the embryo of the age 

 under consideration is influenced through any medium by its 

 entodermal surfaces. The mouth is not formed till long after the 

 period and the entodermal surfaces are not accessible to the typi- 

 cal stimuli of later life. The cloacal aperture of the archenteron 

 is, of course established earlier, and it is conceivable that in the 

 use of soluble substances for stimulating agents, these might 

 diffuse into the cavity at a very slow rate and in exceedingly 



