430 STRUCTURE OF VERTEBRATA. 



were held to correspond to the centrum, the neural arches, and the 

 neural spine of a vertebral body. 



This undoubtedly suggestive theory, modified in various details, per- 

 sisted for a long period, but ultimately gave way before the advances in 

 comparative anatomy and embryology. Pluxley gave it its death blow, 

 and Gegenbaur replaced it by what may be called the segmental theory 

 of the skull. 



To realise this theory, we must go back in development to the period 

 before the mesoblast has ensheathed the notochord. At this time the 

 segmentation of the body is expressed, not in the skeleton (notochord), 

 but in the primitive segments. These segments, though less obvious 

 than in the trunk, are represented in the head region. Formerly nine 

 were enumerated, but it appears that in Elasmobranchs they are more 

 numerous. Subsequently, l^rain and spinal cord become alike enveloped 

 in the mesoblastic sheath, which gives rise to the skeleton of both head 

 and trunk. 



The great development of the muscle segments of the trunk region 

 induces a secondary segmentation of the mesoblastic skeleton (vertebral 

 column), while the slight development of the muscles of the head region 

 exercises no such influence upon its skeleton, this is therefore always 

 quite devoid of segmentation. The segmentation of the head, in contra- 

 distinction to the skull, is expressed, although indistinctly, by the 

 muscle segments and by the nerves supplying these, perhaps also by 

 the lateral sense organs, the ganglia, and the arches, ^^'hile it is quite 

 certain that it is the head that is segmented and not the skull, the 

 details of the segmentation are still much debated. 



[Table. 



