134 G. E. COGHILL 



be concerned in the origin of root fibers in Amphioxus as they 

 certainly are in Amblystoma. 



The further development of the definitive ventral roots prob- 

 ably involves the differentiation of neurones within this primary 

 somatic motor column. The primary condition consists in a 

 slender collateral which arises from a comparatively large axone 

 that has been oriented in the column for some time before the 

 collateral makes its appearance (figs. 14, 20, 7, 9, 4). Neurones 

 entering the root later progressively enlarge their collateral branch 

 at the expense of the main process to the tract (figs. 10, 11, 12, 

 13). This change probably involves the differentiation of neu- 

 rones in the immediate vicinity of the primary root after the 

 collateral has established itself. As the muscular function of the 

 myotome increases neurones arise which devote their entire neu- 

 rite to the formation of the root while their dendrites have only 

 insignificant or a very subordinate part in the longitudinal con- 

 duction. Such a mode of later differentiation is not established 

 in detail but numerous observations render it plausible. 



3. THEORETICAL CONSIDERATIONS 



1. Regulation in the origin of the motor roots 



There is a definite correlation of the outgrowth of the primary 

 root collateral and differentiation within the myotome. The col- 

 lateral always grows to the middle of the muscle cell and applies 

 itself to the cell at the point opposite a large nucleus against 

 which impinges a conspicuous mass of pigment. In the earlier 

 condition of the mesoderm pigment is quite generally distributed 

 as small granules throughout the cells of the myotome. As the 

 cells differentiate into muscle cells this pigment becomes segre- 

 gated into this central mass. This is evidence of some sort of 

 a polarization of the middle of the muscle cell with reference to 

 the ends. The motor fiber invariably grows to this central region 

 of polarization. The exact time relations between the outgrowth 

 of the collateral and the centralization of pigment are not yet 

 determined but the two processes are very distinctly correlated. 



