EARLY ONTOGENETIC PHENOMENA IN MAMMALS. 95 
which has been later evolved.t And yet it is also in Nycti- 
cebus that out of this early primordium the comparatively 
spacious allantois originates, which spreads against the 
diplotrophoblast in the well-known way. However, nothing 
would prevent us, neither in Nycticebus nor in Lacerta men- 
tioned before, to look upon the particular features of the 
formation of their allantois as so many reminiscences of an 
earlier connective stalk. 
I have reason to believe that many hesitate to accept my 
conclusions concerning these embryonic phenomena, because 
for them the derivation of the mammalian blastocyst out of a 
yolk-laden sauropsidan egg, as it has since a long series of 
years been taught in all manuals of embryology, seems too 
well established, all the more so because the Ornithodelphia 
appear to be such typical transitions. 
It is, however, my conviction that in the Ornithodelphia 
as in the Sauropsida—a profusion of yolk and ovi- 
parity have both arisen only after viviparous an- 
eestral forms had preceded, in which a. larval 
envelope (trophoblast) and its respective deri- 
vates (diplotrophoblast, amnion) were already pre- 
sent. Rapid vascularisation of the trophoblast by means of 
umbilical vessels (as it must have existed in those ancestors) 
was replaced in those descendants that obtained a considerable 
increase of yolk by an early vascularisation of the wall of the 
yolk-sac (area vasculosa). Only later the palingenetic vascu- 
larisation of the larval envelope (trophoblast) again comes to 
1 It is certainly striking that both from Corning’s figures (795) of Lacerta 
and from those of Bonnet (’84, 789) of the sheep, it follows that also, 
according to these authors, the allantois appears earlier than the caudal gut; 
and that thus the conception of the allantois as the older, posterior, axially 
situated elongation of the intestine (cf. Hubrecht, 02, xv, figs. 5 and 7), 
that has acquired great importance for the vascularisation of the equally 
primitive connective stalk, appears quite valid. Moreover, by this conception 
the phylogeny and the ontogeny of the allantois are more easily reconciled 
than by that other, which sees in the allantois only a later vesicular formation 
which protrudes ad hoe. 
