24 A. A. W. HUBRECHT. 
The facts above cited force us to the conclusion that, 
before the formation of the amnion in Sphenodon and in 
Chameeleo begins, there must exist on the surface of the 
blastocyst a circular delimitation of a central region—what 
would be the actual embryonic shield of mammals—from a 
peripheral trophoblastic region. This delimitation is clearly 
indicated in another of Schauinsland’s figures (Pl. 46, 
fig. 117) for Sphenodon not reproduced in Hertwig, but 
here reproduced in Fig. 78. In Schauinsland’s text (703, 
p. 142) this is noticed in the following words :—‘ As it was 
repeatedly noticed (the trophoblast-cells) do not spread over 
the embryo proper, and thus the extra-embryonic and the 
embryonic portion of the ectodermal blastoderm can be 
sharply distinguished from each other.” 
If we now restrict ourselves to the three cases here cited, 
a tortoise (Mitsukuri), Sphenodon, and the chameleon 
(Schauinsland), and purposely leave out of consideration all 
Mehnert’s cases, then we have three Sauropsida in which 
clear indications are noticeable that the mammalian tropho- 
blast is after all also present in the Sauropsida. 
Besides these indications there is, however, a strong 
& priori probability that views which are applicable to the 
embryonic membranes of mammals ought also to fit im with 
Sauropsids that have—because of these membranes—always 
stood in closer connection with the mammalia than with the 
lower vertebrates. 
And we should not lose view of the fact that the com- 
parison of Klasmobranch with Sauropsid ontogeny has always 
shown this incisive difference that there was never a mem- 
brana serosa nor an amnion in the former, so that a direct 
comparison in these two types of the process of the gradual 
inclosure of the yolk by radial expansion from the ectodermic 
shield was tainted by suspicion from the beginning: the whole 
of the serous membrane and the amnion being shed at birth 
in birds, reptiles and mammals; these being, in fact, larval 
layers. 
