SOMATIC MOTOR COLUMN OF AMBLYSTOMA 123 



of the more central portion' of the section. Near the periphery 

 of the cord are nuclei of very different form. They are large 

 and approximately spherical. The indistinctly differentiated 

 perikarya to which these nuclei belong (VC) are crowded with 

 yolk spherules, some of which occupy indentations in the nuclei. 

 In fact the nuclei seem to partially surround such spherules in 

 an ameboid fashion. Among these peripheral cells of figure 1 

 are at least two (VC) which clearly belong to the motor column. 

 The more rostral one has a distinctly differentiated descending 

 process (DP) which runs longitudinally immediately within the 

 external limiting membrane of the cord. There is also a sug- 

 gestion of an ascending process. The absence of a clearly differ- 

 entiated ascending process in the case of this neurone is explained 

 by the appearance of the more caudal neurone (VC) which lacks 

 its nucleus and descending process. Obviously the plane of sec- 

 tion is such that a section which passes through the nucleus of 

 a tract cell includes the descending but not the ascending process ; 

 while a section which passes a little ventrad of the nucleus, as 

 the section does in this case, strikes through the ascending proc- 

 ess. Accordingly the section next dorsad of this shows a typical 

 tract cell nucleus in the position that corresponds with this 

 neurone and not less than three others of the same class distrib- 

 uted more caudally in a series. The more caudal neurone of 

 this figure, then, belongs also to the motor column, and from 

 its ascending process arises a branch which passes through the 

 external limiting membrane as a root fiber to the third myotome. 

 In this section this root fiber reaches the anlage of the spinal 

 ganglion, among the cells of which it can be recognized in the 

 adjacent section. 



Since the first post-otic myotome ordinarily has no motor root 

 (a root has been found in at least one instance), the root repre- 

 sented in figure 1 (VF) is the second motor root and is associated 

 with the anlage of the first spinal ganglion. The root to the 

 second myotome in this specimen can also be identified. While 

 the plane of section through it does not permit positive conclu- 

 sions regarding its mode of origin the root fiber certainly either 



