EARLY ONTOGENETIC THENOMENA IN MAMMALS. 231 
in fig. 10, Taf. xvii. When we turn to Dasyurus there is 
still greater difficulty. According to Hill’s (’08) interpreta- 
tion of his specimens the descendant cells of the lower of his 
two pi'imary rings which result from the segmentation of the 
ovum are to be regarded as “ trophoblast,” the descendants 
of the upper ring as epiblast and hypoblast as indicated in 
my diagram, text-fig. 1 B. 
If this is the correct interpretation clearly there is no time 
when the conditions are as all agree them to be in Eutherian 
mammals. 
There are certain fundamental differences between the 
conditions of the segmenting Metatherian and Eutherian 
eggs— if the only two cases so far known of the former are 
typical of the group. Thus, in the iVIetatherian egg in the 
four-segment stage the segments are like those of Amphioxus 
or frog, in one plane, whilst in the Eutherian egg they are 
always eventually in pairs lying across one another, as 11. 
Hertwig points out in his article in O. Hertwig’s ‘ Handbuch,’ 
vol. i. With this is probably correlated another great differ- 
ence, namely, that the result of further segmentation is in 
the Metatheria a hollow blastula, in Eutheria a solid morula. 
Whether Hill is right in calling the lower ring ectoblastic 
trophoblast is perhaps doubtful, but clearly there is no hint 
in his description (‘ Nature,’ ’08, p. 049) of any Embryo- 
nalhiille, any layer lying over the definitive epiblast, at any 
time of segmentation. Hill has not yet published any 
detailed account of the formation of the hypoblast ; but 
there is nothing in what he has published to pievent one 
regarding the trophoblast as of yolk-cell or hypoblastic 
origin. The difference, then, between the Metatheria and 
the Eutheria would in that case be that while in the latter 
the hypoblast and yolk-cell mass overflow the embryonal 
area and give rise to the “ Rauber layer,” to use an old term, 
in the Metatheria it never overflows. So in the Metatheria 
there is neither Rauber layer nor entypie. The condition of 
the opossum is less ea.sy to bring into line with my view unless 
one takes it to have been brought about by a diminution of 
