336 DEVELOPMENT OF ELASMOBRANCH FISHES. 



(3) entirely from the hypoblast, 



(4) or may have no fixed origin. 



The fourth of these possibilities may for the present be 

 dismissed, since it can be only maintained should it turn out 

 that all the other views are erroneous. The first possibility is 

 supported by the case of the Ccelenterata, and we might almost 

 say by that of this group only 1 . 



Amongst the Ccelenterata the mesoblast, when present, is 

 unquestionably a derivative of the epiblast, and when, as is 

 frequently the case, a distinct mesoblast is not present, the 

 muscle-cells form a specialized part of the epidermic cells. 



The condition of the mesoblast in these lowly organized 

 animals is exactly what might ci priori have been anticipated, 

 but the absence throughout the group of a true body-cavity, or 

 specially developed muscular system of the alimentary tract, 

 prevents the possibility of generalizing for other groups, from 

 the condition of the mesoblast in this one. 



In those animals in which a body-cavity and muscular 

 alimentary tract are present, it would certainly appear reasonable 

 to expect the mesoblast to be derived from both the primitive 

 layers : the voluntary muscular system from epiblast, and the 

 splanchnic system from the hypoblast. This view has been 

 taken and strongly advocated by so distinguished an embry- 

 ologist as Professor Haeckel, and it must be admitted, that on 

 d priori grounds there is much to recommend it ; there are, 

 however, so far as I am aware of, comparatively few observed 

 facts in its favour. 



Professor Haeckel's own objective arguments in support of 

 his view are as follows : 



1 The most important other instances in addition to that of the Ccelenterata which 

 can be adduced in favour of the epiblastic origin of the mesoblast are the Bird and 

 Mammal, in which according to the recent observations of Hensen for the Mammal, 

 and Kb'lliker for the Mammal and Bird, the mesoblast is split off from the epiblast. 

 If the views I have elsewhere put forward about the meaning of the primitive groove 

 be accepted, the derivation of the mesoblast from the epiblast in these instances 

 would be apparent rather than real, and have no deep morphological significance for 

 the present question. 



Other instances may be brought forward from various groups, but none of these 

 are sufficiently well confirmed to be of any value in the determination of the present 

 question, 



