ORIGIN OF THE GERMINAL LAYERS. 285 



cavity, as in Eucope (tig. 202), and to lose their cilia. These cells give 

 rise to an internal pai"encliyma, which carries on an intracellular digestion. 

 At a later stage a central digestive cavity is supposed to be formed. This 

 view of the passage from the jirotozoon to the metazoou state, though to 

 my mind improbable in itself, fits in very well with the ontogeny of the 

 lower Hydrozoa. 



Another view has been put forward by myself in the chapter on the 

 Porifera', to the effect that the amphiblastula larva of Calcispongise may 

 be a transitional form between the Protozoa and the Metazoa, composed of 

 a hemis[)hei'e of nutritive amoeboid cells, and a hemisphere of ciliated cells. 

 The absence of such a larval form in the Coelenterata 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 Coelenterata ; and above all, (3) by the remarkable cha- 

 racters 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 erabryological evidence 

 is insufficient to decide whether the amphiblastula larva is, as sug- 

 gested 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 pro- 

 posed 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 stomodieum and proctodamui — are derived from the epiblast ; 



1 Vol. I. p. 122. 



