ORIGIN OF THE GERMINAL LAYERS. 345 



arguments for regarding the Porifera as a phylum of the 

 Metazoa derived independently from the Protozoa. This seems 

 to me to be shewn (i) 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 Ccelenterata ; 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 stomodaeum and proctodaeum 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 stomodaeum and proctodaeum give 



