118 DERIVATION OF THE MESOBLAST. 



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



(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 Coelenterata, and we might almost 

 say by that of this group only\ 



Amongst the Coelenterata the mesoblast, when present, is 

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

 (piently 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 d 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 

 a iwiori 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) From the fact that some investigators derive the meso- 



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

 ^Yhich 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 Kolliker 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 gi'oove 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 arc sufficiently well confirmed to be of any value in the determina, 

 iioii uf the present question. 



