260 
F. M. BALFOUR.' 
the same time there appear to be strong arguments for re- 
garding the Porifera as a phylum of the Metazoa derived 
independently from the Protozoa. This seems to me to be 
shown (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 re- 
markable 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. Whether or no the amphiblastula larva is, as 
suggested above, to be regarded as the larval ancestor of the 
Porifera must be left as an open question. 
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 propounded. 
Since there are some Metazoa with only two germinal 
layers, and other Metazoa with three, and since, as is shown 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 w’ith 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 epi- 
blast 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 formed 
of epiblast. In some of the Isopods, for example, the 
stomodeeum and proctodeeum give rise to almost the whole 
of the alimentary canal with its appendages, except tha 
liver. 
The origin of the Mesoblast. — The 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 pro- 
