342 DEVELOPMENT OF ELASMOBRANCH FISHES. 



the alimentary tract in a manner almost identically the same as 

 in Sagitta. 



It thus appears that, so far as can be determined from the 

 facts at our disposal, the mesoblast in almost all cases is derived 

 from the hypoblast, and in three widely separated groups it 

 arises as a pair of diverticula from the alimentary tract, each 

 diverticulum containing a cavity which eventually becomes the 

 body-cavity. I have elsewhere suggested 1 that the origin of 

 the mesoblast from alimentary diverticula is to be regarded as 

 primitive for all higher animals, and that the more general cases 

 in which the mesoblast becomes split off, as an undivided layer, 

 from the hypoblast, are in reality derivates from this. The 

 chief obstacle in the way of this view arises from the difficulty of 

 understanding how the whole voluntary muscular system can 

 have been derived at first from the alimentary tract. That part 

 of a voluntary system of muscles might be derived from the con- 

 tractile diverticula of the alimentary canal attached to the body- 

 wall is not difficult to understand, but it is not easy to believe 

 that the secondary system so formed could completely replace 

 the primitive muscular system, derived, as it must have been, 

 from the epiblast. In my paper above quoted will be found 

 various speculative suggestions for removing this difficulty, 

 which I do not repeat here. If it be granted, however, that 

 in Sagitta, Brachiopods, and Echinoderms we have genuine 

 examples of the formation of the whole mesoblast from ali- 

 mentary diverticula, it is easy to see how the formation of the 

 mesoblast in Vertebrates may be a secondary derivate from an 

 origin of this nature. 



An attempt has been already made to shew that the meso- 

 blast in Elasmobranchs is formed in a very primitive fashion, 

 and for this reason the Elasmobranchs appear to be especially 

 adapted for determining whether any signs are exhibited of a 

 derivation of the mesoblast as paired diverticula of the ali- 

 mentary tract. There are, it appears to me, several such 

 features. In the first place, the mesoblast is split off from the 

 hypoblast not as a single mass but as a pair of distinct masses, 

 comparable with the paired diverticula already alluded to. 



1 Comparison of Early Stages, Quart. Jl. Micros. Science, July, 1875. [This 

 Edition, No. VI.] 



