126 SEGMENTATION OF THE VERTEBRAL BODIES, 



and Ampliiura, are stated to be solid at first and only sub- 

 sequently to become hollow. Should the accuracy of Metsch- 

 nikoff 's account of this point be confirmed, an almost exact paral- 

 lel to what has been supposed by me to have occurred with the 

 mesoblast in Elasmobranchs, and other groups, will be supplied. 



The tendency of our present knowledge appears to be in 

 favour of regarding the body-cavity in Vertebrates as having 

 been primitively the cavity of alimentary diverticula, and the 

 mesoblast as having formed the walls of the diverticula. 



This view, to say the least of it, suits the facts which we 

 know far better than any other theory which has been pro- 

 posed, and though no doubt the a prioi^i difficulties in its way 

 are very great, yet it appears to me to be sufficiently strongly 

 supported to deserve the attention of investigators. In the 

 meantime, however, our knowledge of invertebrate embryology 

 is so new and imperfect that no certainty on a question like 

 that which has just been discussed can be obtained ; and any 

 generalizations made at present are not unlikely to be upset by 

 the discovery of fresh facts. 



The only other point in connection with the mesoblast 

 which I would call attention to is the formation of the ver- 

 tebral bodies. 



My observations confirm those of Remak and Gegenbaur, 

 shewing that there is a primary segmentation of the vertebral 

 bodies corresponding to that of the muscle-plates, followed by a 

 secondary segmentation in which the central lines of vertebral 

 bodies are opposite the partitions between the muscle-plates. 



The explanation of these changes is not difficult to find. 

 The primary segmentation of the body is that of the muscle- 

 plates, which must have been present at a time when the 

 vertebral bodies had no existence. 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 conditions under which the lateral muscles can cause 

 the flexure of the vertebral column are clearly that each 

 muscle- segment shall be capable of acting on two vertebrae; 

 and this condition can only be fulfilled when the muscle- 

 segments are opposite the intervals between the vertebrae. 

 Owing to this necessity, when the vertebral segments became 



