VII.] THE PERMANENT VERTEBRA. 205 



blast, still marked, however, by the transparent lines 

 indicating the fore and aft limits of the several somites. 

 This mesoblastic investment is sometimes spoken of as 

 the "membranous" vertebral column. 



The portions of the somites thus forming the primary 

 vertebrae or membranous vertebral column are converted 

 into the permanent vertebrae; but their conversion is 

 complicated by a remarkable new or secondary segmen- 

 tation of the whole vertebral column. 



On the fourth day, the transparent lines marking 

 the fore and aft limits of the somites are still distinctly 

 visible. On the fifth day, however, they disappear, so 

 that the whole vertebral column becomes fused into a 

 homogeneous mass whose division into vertebrae is only 

 indicated by the series of ganglia. This fusion, which 

 does not extend to the muscle-plates in which the 

 primary lines of division still remain visible, is quickly 

 followed by a fresh segmentation, the resulting segments 

 being the rudiments of the permanent vertebrae. The 

 new segmentation, however, does not 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 muscle- 

 plate shall be capable of acting on two vertebrae ; and this con- 

 dition can only be fulfilled when the muscle-segments are oppo- 



