SOME PROBLEMS OF COELENTERATE ONTOGENY 533 
than formerly, one still finds it more or less dominant in embryol- 
ogy. Inhis book, ‘The Development of the Frog,’ Morgan (’97) 
gives the subject usual attention; and in the still more recent 
book, ‘The Development of the Chick,’ Lillie (08) devotes several 
pages to the subject, and it crops out repeatedly in the earlier 
chapters. The germ layer theory came to have a larger place 
than might otherwise have been the case in the attempt to dis- 
cover some ultimate embryological basis for homology, and simi- 
lar warrant for the so-called Biogenetic Law, or Doctrine of Reca- 
pitulation. 
It has long passed as a cardinal doctrine in embryology that 
the primary germ layers form a constant, and more or less infal- 
lible basis for homology,—a sort of court of last appeal where 
other criteria fail. But not a few recent results have tended to 
force the concession that even here there have been hasty gener- 
alizations. Not only in modes of formation and development 
have the germ-layers been found to differ widely, but in their 
function and fate in ontogeny there has likewise been obvious 
variation and discrepancy at many points. Balfour long ago 
called attention to discrepant modes of mesoderm formation, 
and recent experimental results have shown that organs of usual 
ectodermic origin are far from dependent on such mode of deriv- 
ation. In coelenterate ontogeny the most radical divergences 
as to modes of origin are too well known to call for extended review. 
From the extreme mode of delamination exclusively in entoderm 
formation as pointed out by Metschnikoff in Geryonia, and since 
confirmed in substance by Brooks (’86), who ealls it “ very peculiar, 
and without any exact parallel,’ to that of gastrulation in Sey- 
phozoa, with its confusing variations and exceptions, which invol- 
ved those rancorous discussions of Claus, Goette, and others, 
and the more usual mode through the morula, the entire gamut 
of germ-layer formation might seem to be epitomized. But 
despite the misguided and essentially mischievous (however well 
meant), efforts to derive all these phases from a mythical gastraea, 
now long a discredited and discarded phylogenetic monstrosity, 
the fact remains that there is probably no genetic relation what- 
soever between them. 
