SPINAL CORD AND BULBO-SPINAL JUNCTION 127 



these ordered movements are effected remain obscure. At the incep- 

 tion of motihty in the embryo the first neuromotor responses to 

 stimulation are mass movements, and the apparatus of local reflexes 

 matures later (Coghill, '29). This implies that integrative functions 

 of total-pattern type mature earlier than do the partial patterns of 

 the local reflexes. Coghill's studies revealed a transitory system of 

 peripheral and central connections in the early stages of the develop- 

 ment of motility when mass movements prevail, followed by radical 

 changes as the action system becomes more complicated. 



Before the spinal ganglia are functionally mature, a series of transi- 

 tory giant ganglion cells (Rohon-Beard cells) within the cord send 

 peripheral processes out to skin and myotomes and central processes, 

 which effect connection with the neuromotor elements. The transi- 

 tory cells subsequently disappear and are replaced by the more spe- 

 cialized elements of the spinal ganglia. 



Intramedullary cells of sensory type were observed by Humphrey 

 ('44) in the spinal cords of human embryos. Two types of bipolar 

 sensory cells appear in embryos of 5 mm., one of which is transient 

 and is regarded as homologous with the Rohon-Beard cells. The other 

 type persists to functional stages, and at the beginning of motility 

 {'i''2.5 mm.) many of these are changing to a unipolar shape and re- 

 semble cells of the spinal ganglia. These intramedullary unipolar 

 cells are found in embryos of from 16 to 144 mm. in length, and are 

 regarded as functioning components of the dorsal roots in the early 

 stages of motility. Youngstrom ('44) also reports the occurrence of 

 sensory cells within the spinal cords of human embryos of from 19 to 

 63 mm. These cells are in the mantle layer and resemble those re- 

 garded by Humphrey as comparable with Rohon-Beard cells. Similar 

 intramedullary cells of sensory type have been seen by many others 

 in embryos and adults to accompany root fibers of spinal and cranial 

 nerves (see Pearson, '45, for illustrations) ; and it is probable that 

 these are all derived from the neural crest, like the mesencephalic 

 nucleus of the V nerve (as Piatt, '45, has demonstrated). 



On the motor side of the arc two types of peripheral neurons were 

 described by Coghill ('26, Paper VI) and Youngstrom ('40) : (1) The 

 thick primary fibers appear first in ontogeny and course for long 

 distances in the ventral funiculus before emergence. The first ventral 

 root fibers arise as collaterals of these longitudinal axons. (2) Thinner 

 secondary fibers, which appear later, pass out from the gray of the 

 cord more nearlv transverselv- The Rohon-Beard cells are centrally 



