Another view has been put forward by myself in the chapter on the Porifera[123], to the effect that the amphiblastula larva of Calcispongiæ may be a transitional form between the Protozoa and the Metazoa, composed of a hemisphere of nutritive amœboid cells, and a hemisphere of ciliated cells. The absence of such a larval form in the Cœlenterata and higher Metazoa is opposed, however, to this larva being regarded as a transitional form, except for the Porifera.
It is obvious that so long as there is complete uncertainty as to the value to be attached to the early developmental processes, it is not possible to decide from these processes whether there is only a single metazoon phylum or whether there may not be two or more such phyla. At the same time there appear to be strong arguments for regarding the Porifera as a phylum of the Metazoa derived independently from the Protozoa. This seems to me to be shewn (1) by the striking larval peculiarities of the Porifera; (2) by the early development of the mesoblast in the Porifera, which stands in strong contrast to the absence of this layer in the embryos of most Cœlenterata; and above all, (3) by the remarkable characters of the system of digestive channels. A further argument in the same direction is supplied by the fact that the germinal layers of the Sponges very probably do not correspond physiologically to the germinal layers of other types. The embryological evidence is insufficient to decide whether the amphiblastula larva is, as suggested above, to be regarded as the larval ancestor of the Porifera.
Homologies of the germinal layers. The question as to how far there is a complete homology between the two primary germinal layers throughout the Metazoa was the third of the questions proposed to be discussed here.
Since there are some Metazoa with only two germinal layers, and other Metazoa with three, and since, as is shewn in the following section, the third layer or mesoblast can only be regarded as a derivative of one or both the primary layers, it is clear that a complete homology between the two primary germinal layers does not exist.
That there is a general homology appears on the other hand hardly open to doubt.
The primary layers are usually continuous with each other, near one or both (when both are present) the openings of the alimentary tract.
As a rule an oral and anal section of the alimentary tract—the stomodæum and proctodæum—are derived from the epiblast; but the limits of both these sections are so variable, sometimes even in closely allied forms, that it is difficult to avoid the conclusion that there is a border-land between the epiblast and hypoblast, which appears by its development to belong in some forms to the epiblast and in other forms to the hypoblast. If this is not the case it is necessary to admit that there are instances in which a very large portion of the alimentary canal is phylogenetically an epiblastic structure. In some of the Isopods, for example, the stomodæum and proctodæum give rise to almost the whole of the alimentary canal with its appendages, except the liver.
The origin of the Mesoblast. A diploblastic condition of the organism preceded, as we have seen, the triploblastic. The epiblast during the diploblastic condition was, as appears from such forms as Hydra, especially the sensory and protective layer, while the hypoblast was the secretory and assimilating layer; both layers giving rise to muscular elements. It must not, however, be supposed that in the early diploblastic ancestors there was a complete differentiation of function, but there is reason to think that both the primary layers retained an indefinite capacity for developing into any form of tissue[124]. The fact of the triploblastic condition being later than the diploblastic proves in a conclusive way that the mesoblast is a derivative of one or both the primary layers. In the Cœlenterata we can study the actual origin from the two primary layers of various forms of tissue which in the higher types are derived from the mesoblast[125]. This fact, as well as general à priori considerations, conclusively prove that the mesoblast did not at first originate as a mass of independent cells between the two primary layers, but that in the first instance it gradually arose as differentiations of the two layers, and that its condition in the embryo as an independent layer of undifferentiated cells is a secondary condition, brought about by the general tendency towards a simplification of development, and a retardation of histological differentiation[126].
The Hertwigs have recently attempted (No. [271]) to distinguish two types of differentiation of the mesoblast, viz. (1) a direct differentiation from the primitive epithelial cells; (2) a differentiation from primitively indifferent cells budded off into the gelatinous matter between the two primary layers.
It is quite possible that this distinction may be well founded, but no conclusive evidence of the occurrence of the second process has yet been adduced. The Ctenophora are the type upon which special stress is laid, but the early passage of amœboid cells into the gelatinous tissue, which subsequently become muscular, is very probably an embryonic abbreviation; and it is quite possible that these cells may phylogenetically have originated from epithelial cells provided with contractile processes passing through the gelatinous tissue.