DEVELOPMENT OF ELASMOBRANCH FISHES. 117 



Osseous Fishes are stated to agree with Amphibians in the 

 development of their proto vertebrae and muscular system \ but 

 further observations on this point are required. 



Though the development of the general muscular system 

 and muscle-plates does not, according to existing statements, 

 take place on quite the same type throughout the Vertebrate 

 sub-kingdom, yet the comparison which has been instituted 

 between Elasmobranchs and other Vertebrates appears to prove 

 that there are one or two common features in their development, 

 which may be regarded as primitive, and as having been inherited 

 from the ancestors of Vertebrates. These features are (1) The 

 extension of the body-cavity into the vertebral plates, and 

 subsequent enclosure of this cavity between the two layers 

 of the muscle-plates ; (2) The primitive division of the vertebral 

 plate into a somatic and a splanchnic layer, and the formation 

 of a large part of the voluntary muscular system out of the 

 splanchnic layer. 



The ultimate derivation of the mesoblast forms one of 

 the numerous burning questions of modern embryology, and 

 there are advocates to be found for almost every one of the 

 possible views the question admits of. 



All who accept the doctrine of descent are agreed that 

 primitively only two embryonic layers were present — the 

 epiblast and the hypoblast — and that the mesoblast subse- 

 quently appeared as a distinct layer, after a certain com- 

 plexity of organization had been attained. 



The general agreement stops, however, at this point, and 

 the greatest divergence of opinion exists wdth reference to all 

 further questions which bear on the development of the meso- 

 blast. There appear to be four possibilities as to the origin of 

 this layer. 



It may be derived : 



(1) entirely from the epiblast, 



responded with the early-formed muscles of Elasmobranchs, and the remaining 

 cells of both layers of the protovertebrae became in the course of development 

 converted into muscle-cells indistinguishable from those formed at first. Is it 

 possible that, owing to the distinctness of the first-formed mass of muscle, Dr 

 Gotte can have overlooked the fact that its subsequent growth is carried on at 

 the expense of the adjacent cells of the somatic layer? 



1 Ehrlich, Ueber den peripher. Theil d.Urwirbel. Archivf. Mic. Anat. Vol. xi. 



