126 G. E. COGHILL 



tions in the cord are shown in figure 8. In this case both the 

 descending and ascending processes can be readily followed for 

 some distance, and the ascending process, presumably the den- 

 drite, is branched. There appears, also, to be a small branch 

 off of the descending process (DP) near its base, while the main 

 process ends abruptly and in such relations as to indicate that 

 it actually extends a considerable distance beyond the limits of 

 this section. The tangential section of the nucleus of this neu- 

 rone, as shown in the figure, is in keeping with the form of the 

 tract cell as demonstrated in other planes of section, which show 

 the neurones with their perikarya shunted off to one side of the 

 main fibrillar axis. The distribution of the yolk spherules in 

 accordance with the fibrillar structure around the nucleus, as 

 shown in this figure, is typical. 



There is no evidence of ventral roots in this embryo at the 

 level of the cell of figure 7; and the general appearance of the 

 myotomes of that level as shown in figure 8 would not lead one 

 to expect ventral roots at this level. However, a study of the 

 two adjacent myotomes throughout their entire extent in the 

 serial sections reveals a few cells in their most ventro-mesial por- 

 tion that might have muscular function. Comparative studies 

 of embryos, on the other hand, lead me to believe that at this 

 stage the ventral root system has not extended so far caudad, and 

 that this tract cell, therefore, lies considerably caudad of any 

 ventral root. 



Another case of the origin of a ventral root fiber as a collateral 

 from a tract neurone in an embryo of this age is shown in figure 

 9. Here the plane of section is in general vertical, but the embryo 

 is twisted in such a way that at the level of this root the section 

 is very nearly in the frontal plane, being tipped considerably 

 latero-ventrad on the side figured. A careful study of this root 

 fiber (VF) by varying the focal plane gives one the impression 

 that the fiber from which the collateral arises is a descending 

 process of a tract neurone, and the angles subtended by the 

 collateral and the axone corroborate this interpretation. Here 

 again is seen the characteristic outward thrust of the axone at 

 the point of origin of the root collateral. 



