EAELY ONTOGENETIC PHEN05IENA IN MAMMALS, 227 
made it sufficiently clear that they do not represent any 
particular animal, hut that they are to be regarded as 
generalised diagrams representing three plausible interpre- 
tations of the observed facts of Eutherian early develop- 
ment. The first was suggested by van Beneden’s papers on 
the rabbit and bat. In this I ought to have shown the 
epiblast thickened at the embryonic pole from the first, 
because van Beneden lays stress on the fact that the inner 
mass contains from the first the embryonic epiblast. As 
drawn it is a compromise between van Beneden’s and Duval’s 
account of the bat. 
But to return for a moment to the three alternative 
suggested explanations of the epibole. The first alternative 
would satisfy Hubrecht’s hypothesis so far as the trophoblast 
of Eutherian mammals is concerned, but how can he accept 
the explanation if, as he desires to do, he regards this 
mammalian trophoblast as the homologue of the epidermic 
layer of epiblast in the Amphibian, or the outer coat and 
periblast of Teleostean, or certain superficial layers in 
Sauropsida ? In all those cases the layer in question arises 
later and by delamiuation, as an investing sheath. There is 
no hint of a growth round an inner mass. If homologous, is 
it not sti’ange that the mammalian trophoblast should be 
formed by epibole ? That is to say, Hubrecht cannot well 
accept this explanation of the epibole, as it would be 
inconsistent with the rest of his theory. 
Other objections to Hubrecht’s view are that it does not 
give a satisfactory explanation of the phenomena known as 
entypie, nor for the rejection of the trophoblast cells by the 
epiblast of the embryonal area (pig, rabbit, mole, etc.), 
whereas if, as the tliiid alternative requires, the trophoblast 
has had a yolk -mass or hypoblast origin in evolution the 
rejection is natural enough. Van Beneden’s view of the 
epibole is plausible, but this again does not satisfactorily 
ex2)lain the rejection of the trophoblast cells by the embryonal 
area, nor does it really explain entypie; and it is not 
su|-)ported by the nature of the epibole suggested by the 
