130 G. E. COGHILL 



ing and descending processes, relations that are clearly demon- 

 strated in other planes of section. For instance, in figure 20, 

 taken from an embryo of the early swimming stage, is drawn a 

 neurone of the motor column at the level of the thirtieth myo- 

 tome. The plane of section here is approximately frontal, but 

 tipped slightly ventro-caudad and dorso-laterad on the side fig- 

 ured. The entire section of the thirtieth myotome is also shown 

 in the figure, and the section through the skin as well. The 

 broad body of the cell, with its characteristic enclosure of yolk 

 spherules, projects laterad against the external limiting membrane 

 and extends caudad in a slender descending process (DP). The 

 basal portion of its ascending process is also perceptible. The 

 position of this neurone of the motor column is considerably 

 caudad of any demonstrable nerve roots. 



At the level of the twelfth myotome of an animal of the same 

 degree of development the mode of origin of both the ventral 

 roots is perfectly clear and the conditions are reproduced in figures 

 24 to 26, inclusive. In figure 24 a number of fibers sweep from 

 cephalad out of the latero- ventral tract (VT) into the ventral 

 root which passes directly into the anlage of the spinal ganglion 

 (SG). One of these fibers {DP), isolated from the others, shows 

 the branched condition and the collateral (VF) to the root. The 

 characteristic loop made by the fibers of the tract at the origin 

 of the root is better shown in the root of the opposite side, as 

 drawn in figure 25. In the section of figure 24 the caudal arm 

 of the loop is cut away excepting for a few scattering fibers. 

 In figure 25 both the rostral and caudal arms of the loop are 

 clearly seen, and one of the fibers of the loop shows the branched 

 condition distinctly. This fiber is drawn alone in figure 26, where 

 the root fiber (VF) is seen as a collateral of a fiber which con- 

 tinues caudad (DP) in the latero-ventral tract. A neurofibril is 

 shown going into each of these divisions of the fiber. In figure 

 24 there is apparently a branching of a single neurofibril with 

 one branch going into each of the processes, but in this, and in 

 other similar cases, two fibrils may be in very close apposition 

 with one another so that their separation in the given plane of 

 section appears like branching. Such appearance occurs also in 

 figures 21, 22 and 27. 



