388 THE ORIGIN OF VERTEBRATES 



However many may have been the original number of segments 

 belonging to the spinal region, one thing is certain — the segmental 

 character of this region is remarkably clearly shown, not only by the 

 presence of the segmental spinal nerves, but also by the marked 

 segmentation of the mesoblastic structures. The question, therefore, 

 that requires elucidation above all others is the origin of the spinal 

 mesoblastic segments, i.e. of the ccelomic cavities of the trunk-region, 

 and the structures derived from their walls. 



Proceeding on the same lines as in the case of the cranial 

 segments, it is necessary in the first instance to inquire of the verte- 

 brate itself as to the scope of the problem in this region. In addition 

 to the variability in the number of segments so characteristic of the 

 spinal region, the complete absence in each spinal segment of a 

 lateral root affords another marked difference between the two 

 regions. Here, except, of course, at the junction of the spinal and 

 cranial regions, each segmental nerve arises from two roots only, 

 dorsal and ventral, and these roots are separately sensory and motor, 

 and not mixed in function as was the lateral root of each cranial 

 segment. Now, these lateral roots were originally the nerves sup- 

 plying the prosomatic and mesosomatic appendages with motor as 

 well as sensory fibres. The absence, therefore, of lateral roots in the 

 spinal region implies that in the vertebrate none of the musculature 

 belonging to the metasomatic appendages has remained. Conse- 

 quently, as far as muscles are concerned, the clue to the origin of 

 the spinal segments must be sought for in the segmentation of the 

 body-muscles. 



Here, in contradistinction to the cranial region, the segmentation 

 is most marked, for the somatic spinal musculature of all vertebrates 

 can be traced back to a simple sheet of longitudinal ventral and 

 dorsal muscles, such as are seen in all fishes. This sheet is split 

 into segments or myotomes by transverse connective tissue septa or 

 myo-commata ; each myotome corresponding to one spinal segment. 



In addition to the evidence of segmentation afforded by the body- 

 musculature in all the higher vertebrates, similar evidence is given 

 by the segmental arrangement of parts of the supporting tissue to 

 form vertebrae. Such segments have received the name of sclerotomes, 

 and each sclerotome corresponds to one spinal segment. 



Yet another marked peculiarity of this region is the segmental 

 arrangement of the excretory organs. Just as our body-musculature 



