20 A. A. W. HUBRECHT. 
to be formed—the remains of the cleavage cells not as yet 
arranged in regular layers. I think we may safely compare 
this stage in the Ornithodelphian development with that of 
the higher mammals in which the, as yet undifferentiated 
embryonic knob is covered by the trophoblast, which has 
dilated into a vesicle. Although the interpretation here given 
differs from that of Semon, I feel confident that further and 
more detailed researches on the development of Monotremes 
will confirm this hypothesis, as well as the supplementary 
one which at present is not yet based on observation, viz. 
that the cells e.k. in Figs. 67 and 69, after a time arrange 
themselves into embryonic ectoderm and entoderm, the latter 
spreading out radially below the trophoblastic cell-layer, as 
indicated in Fig. 70. It is particularly to be regretted that 
the embryonic shield belonging to Semon’s Fig. 39 has come 
to grief, because it would no doubt have settled the point 
here under discussion.! 
The difference between the Ornithodelphia on one side 
and between Mono- and Di-delphia on the other would—if 
the interpretation here given were to be confirmed—consist 
in the fact that the trophoblast vesicle of the former includes 
besides an embryonic knob a very considerable amount of 
food-yolk, the development of which will have gone parallel 
with the change in the ancestral line from viviparity to ovi- 
parity. 
Also in the Sauropsida similar phenomena must have 
occurred of which, however, the traces are yet more difficult 
to establish than was the case in Ornithodelphia. The 
trophoblastic vesicle, which is in Ornithodelphia yet com- 
paratively distinct, though as yet imperfectly known, is in 
many reptiles and birds distinguished with great difficulty 
from the embryonic shield because the phenomenon of the 
trophoblastic vesicle opening up at one spot, in order to let 
1 When this paragraph was first written Wilson and Hill’s latest extensive 
researches (’07) had not yet come into my hands, Their figures, here repro- 
duced in the Figs. 68 and 70, seem to-fully agree with the hypothetical 
interpretation here given, before these new facts had come to light. 
