SEGMENTATION 141 



segments from it are physiologically merely a con- 

 tinuation in later developmental stages of a process 

 essentially similar to that which in the very early stages 

 gives rise to the cephalic neuromeres. Moreover, the 

 very early appearance of indications of segmentation in 

 the primordium of the nervous system, which is ecto- 

 dermal, favors the conclusion that segmentation is 

 primarily an ectodermal rather than a mesodermal 

 process and this conclusion again is in accord with the 

 fact that the ectoderm is physiologically the most 

 active body-layer in the earlier embryonic stages and 

 the layer in which the earliest differentiations appear. 

 The mesodermal theory of segmentation is based pri- 

 marily on morphological speculation rather than on 

 physiological experiment, and it seems to me to offer 

 little prospect of interpretation or synthesis of the 

 various facts as they now stand, and particularly of 

 the very early participation of the ectoderm and its 

 earliest definitive differentiation the nervous primor- 

 dium in the repetitive process. The facts apparently 

 indicate that segmentation begins in the ectoderm and 

 that the processes in this the most active and most 

 advanced body layer affect conditions in other layers. 

 The segmental ganglion is then the primary definitive 

 organ of the segment as the cephalic ganglion is of the 

 individual. 



