224 
ErCHARD ASSHETON. 
entirely different, and, in many respects, more consistent 
explanation of the origin of the mammalian trophoblast, as 
lias already been urged by other Avorkers on the embryology 
of mammals. 
But what does Hubrecht say with reference to these cases? 
“ Tlie so-called metagastrula stage of mammals, first described 
by van Beneden (’80), has since been abandoned by that 
author (though taken up again by Duval [’99, p. 64]).” It 
is quite true that van Beneden has abandoned his explanation 
offered in 1880 of the epibole, which he described as occurring 
in the rabbit, but he has not I’enounced his faith in the fact, 
but has reiterated it (’99) and described a similar pheno- 
menon in the bat, V. murinus, as Duval also has done 
(Duval, ’99). But although DuA'al supports the metagastrula 
theory (one, however, Avhich is almost certainly untenable), 
van Beneden gives a new, and, to my mind, a much more 
plausible explanation. And at the same time, discussing 
Hubrecht’s theory, van Beneden says of it that “ I’hypothese 
de Hubrecht heurte a des difficultes morphologiques et 
physiologiques insurmontables ; elle laisse inexpliquee I’exist- 
ence, chez les Mammiferes placentaires, d’une vesicule 
ombilicule et d’une foule de caracteres communs ii tous les 
Amniotes et distinctifs de ces animaux” (p. 333). 
Hubrecht attempts later to meet some of the objections 
referred to by van Beneden (also in his former paper, 1902), 
but he nowhere now discusses the question and significance 
of" the epibole. Does he deny that it may occur? 
To me it seems that the evidence in its favour is too strong 
for the possibility to be ignored. Evidence for its occurrence 
rests on the observations of van Beneden on the rabbit, van 
Beneden and Duval on the bat, and myself on the sheep, but 
it may as Avell be admitted at once that it is an extremely 
difficult matter upon which to come to an unhesitating 
opinion, because in many cases, as, for example, the pig, 
phenomena Avhich are so strikingly apparent in the bat and 
sheep are not to be seen at all. 
There can be no doubt, however, that in the bat and sheep 
