122 G. E. COGHILL 



response to tactile stimulation. In order to obtain specimens 

 that are approaching the stage of earliest reaction to touch my 

 method has been to apply the current from an iilductorium gauged 

 so as to stimulate a slightly older individual without injury. 

 This test differentiates the embryos that are very near to the 

 stage in which response to tactile stimulation becomes possible, 

 for the somatic muscles can be stimulated by electricity in this 

 manner for a brief period before they can be stimulated through 

 the tactile receptors. Embryos in this phase of development rep- 

 resent the earliest stage taken account of in this study. Following 

 this, in the natural order of development, are specimens of the 

 early flexure stage (which contract only the most rostral myo- 

 tomes), specimens of the 'coil-reaction' stage (which bend the 

 trunk into a tight coil), specimens in the 'S-reaction' stage (which 

 perform a compound flexure or sinuous movement) and speci- 

 mens of the early swimming stage. The latter specimens are 

 approximately 7 mm. long. Drawings and descriptions of the 

 root fibers of all these stages and remarks upon the methods of 

 fixation and preparation are given in connection with the expla- 

 nation of the figures. The scope of the paper is in no sense 

 intended to cover or take account of the literature upon the sub- 

 ject. This part of the work must be deferred till the publication 

 of the general results of my studies upon correlation of growth 

 and function. 



1. CONDITIONS FOUND IN PARTICULAR CASES 

 1. Embryos reacting to electricity hut not to touch 



The conditions of the root fibers as they occur in the youngest 

 embryos of my collection, that is to say, in embryos which react 

 to electrical but not to tactile stimulation, are illustrated in 

 figures 1 and 2. The plane of section in figure 1 is approxi- 

 mately frontal, but tipped slightly ventro-laterad on the side 

 figured. The most ventral portion of the central canal appears 

 in the section, although the figure reaches only about half way 

 from the periphery of the cord to the canal. The more mesial, 

 elongated nuclei of the figure represent the nuclear characteristics 



