402 TRANSACTIONS OF THE CANADIAN INSTITUTE. [VoL VI. 
had a phylogeny, and, it may be remarked, that just as in other struc- 
tures, we find discrepancies between the phylogenetic and ontogenetic 
development, so too we may expect, and undoubtedly do find, discre- 
pancies between the phylogeny and ontogeny of the germ layers. It 
has generally been accepted that the Ccelentera represent a stage in the 
phylogeny of the germ layers, two of them being fully differentiated ; 
indeed, Huxley’s homology of the Ccelenterate ectoderm and endoderm 
with the epiblast and hypoblast of the embryologists may be regarded 
as one of the foundation stones of the germ layer theory. But, after all, 
can we directly homologize the embryological and Ccelenterate layers ? 
Are the Ccelenterate layers morphologically differentiated? It seems to 
me that they are not ; every kind of cell, glandular, muscular, sensory, 
ganglionic, and even nematoblastic, which we find in the ectoderm, 
occurs also in the endoderm. The Ccelentera represent a stage in the 
evolution of the diploblastic condition, rather than the completion of 
that condition, and we are assuming too much when we make a direct 
homology of their ectoderm and endoderm with the epiblast and 
hypoblast of, let us say, a vertebrate embryo. 
I have spoken of only two layers in the Ccelentera, omitting the 
mesoglcea. This term, now generally accepted for the intermediate 
layer of the Ccelentera, is sufficient reason for so doing, since it implies 
a lack of homology of the intermediate layer with the mesoderm of 
higher types. It seems to me, and I have so expressed myself else- 
where, that if we are to seek for a homologue of the Ccelenterate meso- 
gloea in higher forms we must look for it in the limiting membrane which 
occurs just below the ectoderm. Indeed, a comparison of the mesogloea 
with the limiting membrane of certain Polyclades is exceedingly 
instructive. 
If, then, we regard the Ccelentera as presenting merely an approxi- 
mation to a diploblastic condition, the distinction between an ectoder- 
mal and an endodermal origin of any of these parts becomes relatively 
of little moment. And, furthermore, we need not be surprised to find 
that structures which, in certain antimeres, develop from one so-called 
verm layer, may arise from the other in other antimeres. This may be 
the case with the glandular streaks. Both those who have spoken in 
favour of their ectodermal origin and those who have maintained that 
they were endodermal may have right on their side. The observations 
of Goette (1893), and Miss Hyde (1894), have given reasons for believ- 
ing that, in the Scyphomedusz, while the first pair of radial chambers is 
endodermal, the second pair is ectodermal in origin. If such variation 
occurs in this group in connection with such fundamental structures, 
