128 THE BRAIN OF THE TIGER SALAMANDER 



so connected with the primary motor cells as to evoke mass move- 

 ment of the musculature of the trunk in response to adequate stimu- 

 lation of any kind. In subsequent stages central connections between 

 spinal ganglion cells and secondary motor neurons are made, and 

 these are regarded as provision for execution of local reflexes. The 

 primary motor neurons persist in adult Amblystoma. They occur in 

 larval anurans but disappear at metamorphosis (Youngstrom, '38). 

 Humphrey ('44) describes cells in the spinal cords of very young 

 human embryos, which she believes are surviving vestiges of primary 

 motor neurons of amphibian type. 



lu our sections of the adult the ventral spinal roots contain fibers of 

 primary and secondary type. The primary root fibers are thick and 

 heavily myelinated centrally and peripherally. Some of the thinner 

 secondary fibers are well myelinated, and many of them seem to lose 

 their myelin as they emerge from the spinal cord. Coghill ('26, Paper 

 VI, p. 135) reported that in early swimming stages "a single fiber 

 may innervate an entire myotome, and branches of these same fibers 

 form the earliest nerves to limbs and tongue." At this early stage, 

 however, the musculature of the limb bud is still an undifferentiated 

 primordium. Youngstrom confirmed these observations and ex- 

 pressed the opinion that the limb musculature, like that of the trunk, 

 has a double innervation of both primary and secondary fibers; but 

 no details of the distribution of these fibers in the definitive limb 

 musculature are given. More recently, Yntema ('43a, p. 331) says of 

 the primary fibers in larvae of from 1''2 to 19 mm. in snout-anal 

 length that "typically, they supply the myotomic musculature. In 

 addition, fibers of this kind run to muscles of the extremities"; but 

 again details of their distribution in the limb are lacking. In a per- 

 sonal communication he adds: 'T have found evidence for the dis- 

 tribution of primary motor fibers to at least some muscles of the 

 girdles of larvae, and have seen larger fibers which appear to be pri- 

 mary in the limbs themselves." 



In the frog, with an action system very different from that of 

 Amblj^stoma, the development of these nerves shows corresponding 

 differences, for, as mentioned above, Youngstrom ('38) found that 

 larval frogs have primary and secondary fibers like those of Amblys- 

 toma ; but in the adult frog the primary fibers have completely dis- 

 appeared. The opinion expressed by Taylor ('44) that in frog larvae 

 the primary fibers do not enter the limbs may have no bearing on the 

 innervation of limbs in Amblvstoma because of the radical dift'erence 



