55O NEURAL AND H^MAL ARCHES. 



as the somites (fig. 313, Vr), but this segmentation is soon lost, 

 and there is formed round the notochord a continuous sheath of 

 embryonic connective tissue cells, which gives rise to the arches 

 of the vertebrae, the tissue forming the dura mater, the perichon- 

 drium, and the general investing connective tissue. 



The changes which next follow result in what has been 

 known since Remak as the secondary segmentation of the 

 vertebral column. This segmentation, which occurs in all 

 Vertebrata with true vertebrae, is essentially the segmentation 

 of the continuous investment of the notochord and spinal cord 

 into vertebral bodies and vertebral arches. It does not however 

 follow the lines of the segmentation of the muscle-plates, but is 

 so effected that the centres of the vertebral bodies are opposite 

 the septa between the muscle-plates. 



The explanation of this character in the segmentation is not difficult to 

 find. The primary segmentation of the body is that of the muscle-plates, 

 which were present in the primitive forms in which vertebrae had not 

 appeared. As soon however as the notochordal sheath was required to be 

 strong as well as flexible, it necessarily became divided into a series of 

 segments. 



The condition under which the lateral muscles can best cause the 

 flexure of the vertebral column is clearly that each myotome shall be 

 capable of acting on two vertebrae ; and this condition can only be fulfilled 

 when the myotomes are opposite the intervals between the vertebrae. For 

 this reason, when the vertebrae became formed, their centres were opposite 

 not the middle of the myotomes but the inter-muscular septa. 



These considerations fully explain the characters of the secondary 

 segmentation of the vertebral column. On the other hand the primary 

 segmentation (fig. 313) of the vertebral rudiments is clearly a remnant 

 of a condition when no vertebral bodies were present ; and has no greater 

 morphological significance than the fact that the cells of the vertebrae 

 were derived from the segmented muscle-plates, and then became fused 

 into a continuous sheath around the notochord and nervous axis ; till 

 finally they became in still higher forms differentiated into vertebrae and 

 their arches. 



During the stage represented in fig. 28 g, and somewhat 

 before the cartilaginous sheath of the notochord is formed, there 

 appear four special concentrations of the mesoblastic tissue 

 adjoining the notochord, two of them dorsal (neural) and two of 

 them ventral (haemal). They are not segmented, and form four 

 ridges, seated on the sides of the notochord. They are united 



