DERIVATION OF THE MESOBLAST. 335 



Osseous Fishes are stated to agree with Amphibians in the 

 development of their protovertebrae and muscular system 1 , 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 in- 

 herited from the ancestors of Vertebrates. These features are 

 (i) 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 primi- 

 tively only two embryonic layers were present the epiblast 

 and the hypoblast and that the mesoblast subsequently ap- 

 peared as a distinct layer, after a certain complexity of organiza- 

 tion had been attained. 



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

 the greatest divergence of opinion exists with 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, 



(2) partly from the epiblast, and partly from the hypoblast, 



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." Archiv f. Mic. Anat, 

 Vol. XI. 



