12 A. A. W. HUBRECHIT. 
Heape) that some of the Rauber cells become incorporated 
into the embryonic shield has not been well established nor 
been confirmed of late. I incline to believe in their final dis- 
appearance, and wish to call attention to the transition case 
which we may notice, for example, in Tarsius (Hubrecht, 702 ; 
Figs. 49a, b, 50b), where trophoblast cells can remain for yet 
a considerable time attached to the embryonic ectoderm, but 
also finally disappear. In this case the trophoblast opens up 
according to the type prevalent in the second group described 
above, and the permanence of an isolated trophoblast cell on 
the embryonic ectoderm is a matter of chance. 
We may, in concluding this exposition of the varied rela- 
tions in which trophoblast and embryonic ectoderm stand to 
each other in mammals, insist upon the fact that—if we 
except the Ornithodelphia, which will be discussed hereafter, 
and are as yet barely known as far as their early ontogeny is 
concerned?! (Caldwell, ’87; Semon, ’94; Wilson and Hill, ’03) 
all the Didelphia and Monodelphia hitherto investigated 
show at a very early moment the didermic stage out of which 
the embryo will be built up enclosed in a cellular vesicle 
(the trophoblast), of which no particle ever enters into the 
embryonic organisation. 
4. The Mammalian Gastrula. 
The didermic stage of the mammalian blastocyst just 
alluded to fully deserves the name of the “ gastrula” stage 
as I have elsewhere attempted to expound (702, p. 65—7%5 ; 
’05, p. 408). We should bear in mind, as was noticed in the 
introduction, that comparative ontogeny has come to a dead- 
lock when attempting to fit in the mammals into the current 
interpretation of the early development of vertebrates. To 
' Just lately the more extensive paper of Wilson and Hill (’07) has appeared, 
in which figures are given (pl. 2, figs. 4, 5), which allow us to accept quite 
similar arrangements for the Ornithodelphia (see text-figs. 66—70) 
