332 DEVELOPMENT OF ELASMOBRANCH FISHES. 



Though the Mammals are to be looked on as the most 

 differentiated group amongst the Vertebrates, yet in their 

 embryonic history they retain many very primitive features, 

 and, as has been recently shewn by Hensen 1 , present numerous 

 remarkable approximations to the Elasmobranchs. We find ac- 

 cordingly 2 that the primitive lateral plates of mesoblast undergo 

 nearly the same changes in these two groups. In Mammals 

 there is at first a continuous cavity extending through both 

 the parietal and vertebral portions of each plate, and dividing 

 the plates into a somatic and a splanchnic layer : this cavity is 

 the primitive body-cavity. The vertebral portion of each plate 

 with its contained cavity then becomes divided off from the 

 parietal. The later development of these parts is not accurately 

 known, but it seems that the outer portion of each vertebral 

 plate, composed of two layers (somatic and splanchnic) enclosing 

 between them a remnant of the primitive body-cavity, becomes 

 separated off as a muscle-plate. The remainder forms a vertebral 

 rudiment, &c. Thus the extension of the body-cavity into the 

 vertebral portion of the mesoblast, and the constriction of the 

 vertebral portion of the cavity from the remainder, are as 

 distinctive features of Mammals as they are of the Elasmo- 

 branchs. 



In Birds 3 the horizontal splitting of the mesoblast into 

 somatic and splanchnic layers appears, as in Mammals, to extend 

 at first to the summit of the protovertebrae, but these bodies 

 become so early separated from the parietal plates that this 

 fact has usually been overlooked or denied ; but even on the 

 second day of incubation the outer layer of the protovertebrae is 

 continuous with the somatic layer of the lateral plates, and the 

 inner layer and kernel of the protovertebrae with the splanchnic 

 layer of the lateral plates 4 . After the isolation of the proto- 

 vertebrae the primitive position of the split which separated 

 their somatic and splanchnic layers becomes obscured, but when 



1 Zeitschriftf. Anat. Ent-wicklungsgeschichte, Vol. I. 

 8 Hensen loc. cit. 



3 For the history of protovertebrae and muscle-plates in Birds, vide Elements of 

 Embryology, Foster and Balfour. The statement there made that the horizontal 

 splitting of the mesoblast does not extend to the summit of the vertebral plate, must 

 however be regarded as doubtful. 



4 Vide Elements of Embryology, p. 56. 



