(1) entirely from the epiblast,
(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 Cœlenterata, and we might almost say by that of this group only[211].
Amongst the Cœlenterata 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 à 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 embryologist as Professor Haeckel, and it must be admitted, that on à 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) From the fact that some investigators derive the mesoblast with absolute confidence from the hypoblast, while others do so with equal confidence from the epiblast, he concludes that it is really derived from both these layers.