EARLY ONTOGENETIC PHENOMENA IN MAMMALS. 229 
of function, the hypoblast or yolk-cells having- retained their 
function of providing- nourishment for the developing embryo 
throughout the entii-e period of transition from meroblastic 
to holoblastic conditions. 
But of course it involves descent of mammals from large 
yolked eggs of the Sauropsidan type, and is, therefore, so 
diametrically opposed to Hubrecht’s hypothesis that he would 
seem to consider it unworthy of consideration. 
2. Thk Mktathekian and Pkotothekian Blastocyst. 
Hubrecht terminates tlie above discussion with the con- 
clusion that “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.” Leave out the words “ Didel- 
})hia and,” and agreement with the conclusion will be easy 
enough. 
The only cases we have recorded of the earliest stages of 
the Didelphia are that of tlie opossum by Selenka (’87), and 
those of Dasyuriis and Perameles by Hill (’U8). Hubrecht’s 
interpretation of Selenka’s figures is well known from his 
paper on Tarsius (’02), in which (p. 55 et seq.) he argues 
that the outer layer of the vesicle, Pig. 10, Taf. xvii of 
Selenka’s j)aper, is trophoblast, and that the large inner 
ceil, cu., gives rise to the whole embryonic ectoderm and 
endoderm. 
In the accompanying text-figure, fig. C represents Selenka’s 
description of the opossum blastocyst. 
Hubrecht’s interpretation would be legitimate enough if 
we regard the character of the cells as drawn by Selenka 
as only diagrammatic. If, however, Selenka’s sections are 
accurately represented, it is very hard after studying 
Selenka’s figures 2, 3, 4, 8 on Taf. xviii to believe that the 
thickened outer part of the outer layer labelled ex in fig. 4 
has really been derived from the group labelled en in fig. 2 
