anatomical problems bearing upon evolution. 5 



Segmentation. 



It is of one of these characteristics — segmentation — 

 that I wish to speak a few words. We are rather apt, 

 in my opinion, to attach too much importance to this one 

 feature. We are inclined, I think, to regard vertebrate 

 segmentation as a primary feature, whereas although it is 

 doubtless fundamental and essential, it is really a 

 secondary event or process in the architecture of the 

 animal. There is a still more elementary plan of 

 structure in the vertebrate to which the process of seg- 

 mentation is superadded and applied. 



The most primitive plan — the essential architecture 

 — of the vertebrate organism is to be seen in the longi- 

 tudinal series (median or lateral) of structures which con- 

 stitute the organs of the body, the notochord, central 

 nervous system, alimentary canal, vascular and genito- 

 urinary organs. These structures are all in their origin 

 unsegmented, and are only secondarily and partially 

 affected by the segmental process. 



Herbert Spencer makes the luminous suggestion that 

 segmentation has arisen on account of the necessities of 

 life, for feeding or protection ; that lateral cleavage is 

 produced by the stress and strain laid upon a tubular 

 organism in its efforts at locomotion. 



As a matter of fact, in the embryo the segmental 

 process begins with the formation of organs concerned in 

 the production of a locomotor mechanism — the myotomes 

 or muscle-plates — which eventually produce the axial 

 muscles and the axial skeleton. 



Closely connected with, and immediately subsequent 

 to, the formation of these plates, the nerve roots arise 

 from the spinal cord in pairs, and directed between the 

 myotomes retain in their peripheral course a definitely 

 segmental arrangement, which becomes fixed and stereo- 



