128 G. E. COGHILL 



neurones of the 'ventral column and the neurones of the dorsal 

 column, the giant ganglion cells, are differentiated in the most 

 peripheral part. 



The structure of the myotome opposite this root is drawn in 

 figure 15. Careful examination of this myotome and others of 

 the immediate region gives no evidence of their having differ- 

 entiated muscle cells in any part. This fact, together with the 

 fact that in specimens that are physiologically much more ad- 

 vanced in development than that from which this figure was 

 taken, root fibers can be demonstrated only as far caudad as the 

 twenty-seventh myotome, leaves no doubt that, in this level of 

 the cord, the ventro-lateral tract neurones become differentiated 

 and oriented considerably in advance of the development of ven- 

 tral roots. 



4. Embryos of the 'S-reactiori' stage 



A group of neurones of the motor column is shown in figure 16. 

 These neurones are found at the level of the twenty-second myo- 

 tome of an embryo of the 'S-reaction' stage, that is, an embryo 

 that can perform the sinuous, double flexure, but can not perform 

 this movement in series so as to effect locomotion. The plane 

 of section here is approximately frontal, but tipped slightly dorso- 

 laterad on the side figured and also slightly dorso-rostrad. The 

 figure, accordingly, shows the ascending processes {AP) to better 

 advantage than it does the descending processes {DP), though 

 considerable portions of the latter are perceptible in one or two 

 of the neurones. It is possible here to demonstrate the ascending 

 process {AP), presumably the dendrite, of one of these tract 

 neurones for a distance of one-half the length of the myotome. 

 The descending process, my general observations lead me to 

 believe, is ordinarily considerably longer than the ascending 

 process. If this is true, the entire length of one of the tract 

 neurones at this level and in this stage of development must be 

 considerably greater than the length of the myotome. 



The relation of the neurones of the motor column to those of 

 other parts of the cord at this age is shown in figure 17. This 

 figure is taken from a section approximately parallel to the sagit- 



