EAliLY ONTOGENETIC PHENOMENA IN MAMMALS. 225 
and rabbit many specimens show what apparently are stages 
in epibole with such diagrammatic plainness that the proba- 
bility of an epibole cannot be ignored, and although such 
plainness is absent from many, e.g. pig, mouse, dog, etc., 
there is nothing in these cases which prevents a similar inter- 
pretation being placed on them. A difference in staining 
reaction does not become established until a later stage. In 
such cases one can neither affirm nor deny epibole, but my 
point is, there is no evidence against it even in them. In 
Tupaja, according to Hubrecht, the staining differentiation 
does not arise until after epibole has occurred. If Tupaja 
were typical, if the cases of Lepus, Ovis, Vespertilio, were 
only like Tupaja, then Hubrecht’s theory of formation of 
the trophoblast by delamination might be regarded as 
established. 
I know that in 1894 I myself doubted van Beneden’s con- 
tention that epibole occurs iu the rabbit, but the specimens 
of segmenting ova of the sheep which I obtained and 
described iu 1898, being extremely well preserved and in 
excellent condition histologically, were to my mind so con- 
vincing that I was quite converted to the view of van Beneden, 
at least as regards the fact of epibole, although I differed 
from him in the interpretation of the facts. And since that 
time we have had the further evidence of Duval and van 
Beneden derived from their study of Cheiropterau develop- 
ment. If this epibole occurs, that is to say, if thei’e really is 
a growth of one set of segments rouud another set during 
the early stages of the segmentation of the Eutherian 
mammabs egg, it seems to me possible to hold one of three 
quite plausible views. Either it is : 
(1) An early separation of trophoblast and a growth of 
trophoblast cells round the embryonic knob; 
(2) A growth of the epiblast over the yolk mass like the 
sliding of the “extra embryonic” epiblastic edge of the 
blastoderm over the yolk in a bird’s egg, as van Beneden 
suggests (though he does not use the terms “epiblast” and 
“ hypoblast ”) ; or — 
