130 H. V. NEAL 



muscles innervated by two independent nerves, the abducens 

 and the trochlear. This evidence alone, however, does not nec- 

 essarily warrant the conclusion drawn by Dohrn, since each 

 myotome is typically innervated by two somatic motor nerves. 

 Further, the two myotomic divisions of the mandibular cavity 

 are not antero-posterior in their relations, as Dohrn's hypothesis 

 would require, but are dorso-ventral in their relations to each 

 other. 



Since it would be difficult to find any somatic motor nerve 

 which does not divide into at least two branches, this fact does 

 not materially strengthen Dohrn's polymeric assumption. 



The second of Dohrn's trochlear neuromeres is a late second- 

 ary subdivision of the most anterior hindbrain neuromere. From 

 the evidence of a purely topographic relation with a clump of 

 disintegrating neural crest cells, Dohrn infers that the trochlear 

 includes the splanchnic motor niduli of two metameres. Unfor- 

 tunately for this supposition, however, Dohrn is unable to dem- 

 onstrate the presence of these two niduli. He states that no 

 splanchnic motor fibers persist in connection with the second 

 trochlear neuromere. Here again he is in error, since the splanch- 

 nic motor fibers of the ramus mandibularis trigemini have their 

 nidulus in this portion of the hindbrain. These fibers enter the 

 brain as the minor root of the trigeminal. If Dohrn's scheme of 

 segmental relations were correct, two splanchnic motor nerves — 

 his 'second trochlear' and the ramus mandibularis trigemini would 

 have their niduli within the same neuromere — Dohrn's 'second 

 trochlear' neuromere. These nerves actually do have their niduli 

 in a single neuromere (neuromere IH — Dohrn's first and second 

 trochlear) but one has a nidulus in the somatic motor column 

 while the other has a splanchnic motor nidulus. The weakest 

 point in Dohrn's argument is its failure to take into consider- 

 ation the central nidular relations. The argument in favor of 

 the polymerism of the trochlear seems unconvincing, although 

 it is the strongest argument yet advanced with this purpose in 

 view. The argument in favor of the polymerism of the oculo- 

 motor is even less adequate. 



