THE PRIMITIVE MESOBLAST 
35 
figures. The diagrams here given supply an explanation of the facts at 
once simpler and more direct. 
One of the most puzzling features of early primate ova is the pre¬ 
cocious formation of the middle layer cells in the cavity of the blasto¬ 
cyst. The present ovum is not yet early enough to warrant a final 
judgment on the subject, but two new points emerge. The mesoblast 
arises at a still earlier stage than it does in Tarsius, and it completely 
fills the blastocyst. The extra-embryonic coelom seen in Peters’ stage has 
not yet been formed. The probability that the extra-embryonic coelom is 
produced by splitting in the mesoblast therefore becomes a certainty. 
In regard to the origin of this early mesoblast two remarks suggest 
themselves. The attachment of the mesenchyme to the amnio-embryonic 
vesicle more definitely and firmly at one point, may possibly indicate that 
it is arising from the ectoderm and from the same region as it does 
in Tarsius, i.e. from the future hind end of the embryonic plate, where 
the primitive streak will afterwards appear. On the other hand it is not 
impossible that the mesoblast arises contemporaneously with the entoderm 
from the embryonic knob ; that in short the entoderm, even while coming 
into existence, is differentiated into an epithelial lamella which constitutes 
the lining of the yolk sac, and a vascular mesenchyme which is concerned 
in forming the vessels of the connecting stalks and chorion, and the blood 
and blood-vessels of the yolk sac. The study of the sections and of the 
characters of the cells rather inclines one to accept the second alternative, 
which could without much difficulty be reconciled with what is known 
concerning the origin of the vascular mesenchyme in later stages. 
THE PROCESS OF IMBEDDING. 
Having now discussed the characters presented by the present ovum, 
and its position in point of development relative to the earliest stages hitherto 
known, we proceed to consider its relations to the decidua, and the mode 
of implantation. In the absence of earlier stages, it will however be 
necessary to consider at greater length than we have yet done the 
comparative data available on these points. As already explained only those 
