KARLY ONTOGENETIC PHENOMENA IN MAMMALS. 25 
And now that the interpretation of the facts in mammals 
has become comparatively easy (see also Chap. III) we should 
not shrink from resolutely interpreting the Sauropsidan 
development along the same lines. 
A comparison of my own figures for early Erinaceus (’89) and of 
van Beneden’s (99) for early Vespertilio blastocysts with the figures above 
referred to of Schauinsland and Mitsukuri convinces us of the possibility of 
looking upon the double layer outside the formative ectoderm—say of Sphe- 
nodon—as a duplication of the trophoblast. The two mammalian 
genera above mentioned, as also Sorex and others, show a duplication and 
even a more considerable thickening yet of the trophoblast immediately outside 
the embryonic ectoderm. And so it would not be a very strained assump- 
tion to say that in reptiles and birds—in which as we have seen Schauinsland 
admits of a sharp line of demarcation between the trophoblast and the 
embryonic shield on the surface (I. c., p. 142)—both layers that are out- 
side of this line of demarcation are trophoblast-cells separated in an outer flat- 
tened and a deeper columnar layer. Hven of this differentiation in shape the 
mammals offer the counterpart, as is seen, to the left side in Figs. 8 and 8¢ 
of van Beneden’s (’99) early bats and Figs. 35—37 of the hedgehog here given. 
We will, moreover, see in Chap. V that the trophoblast often differentiates 
into two layers that are known as the cytotrophoblast and the plasmodi- 
trophoblast. And so the assumption here advocated would oblige us to 
conclude that, in birds and reptiles, a circular patch of embryonic cells 
was separated—not visibly but potentially—from a peripheral region of 
trophoblast cells just as we have established this for 'lupaja, Tarsius, and 
others, in which—after the embryonic shield has opened out—it is no 
longer possible to distinguish the line of demarcation between trophoblast 
cells and embryonic ectoderm cells, although we have noticed its actual 
existence in the successive ontogenetic stages. In most Sauropsida ontogeny 
would no lenger clearly reveal this difference, but still the mutual relations 
would be the same, and exceptionally favourable cases as here described and 
figured (Clemmys, Sphenodon, Chameleo) would be all the more welcome 
confirmations. 
Physiologically the outer layer of the serosa of Sauropsids is recognised to 
have undoubtedly (see p. 21, footnote) certain properties which we also 
encounter in the proliferating trophoblast of mammals. There is, for example, 
a very marked proliferation in the outer layer of Seps, a viviparous lizard in 
which Studiati, Giacomini (91), and others have described both an allantoi- 
dean and an omphaloidean contact (placentation) between the serosa and the 
maternal tissues, 
Similarly the action of the serosa of the chick in the region where Duval 
has described the ‘‘ organe placentaire ”’ gives rise to the same considerations. 
