344 DEVELOPMENT OF ELASMOBRANCH FISHES. 



Only one more instance of this kind need be cited, taken 

 from the Echinoderms. 



The body-cavity and the mesoblast investing it arise in the 

 case of most Echinoderms as hollow involutions of the alimentary 

 tract, but in some exceptional groups, Ophiura and Amphiura, 

 are stated to be solid at first and only subsequently to become 

 hollow. Should the accuracy of Metschnikoff's account of this 

 point be confirmed, an almost exact parallel to what has been 

 supposed by me to have occurred with the mesoblast in Elasmo- 

 branchs, 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 proposed, 

 and though no doubt the a priori 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 vertebral 

 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 the 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 



