26 
AN EARLY HUMAN OVUM 
upon it—the centrifugal traction of the shrinking mesoblast, and a centri¬ 
petal traction due to the precipitation of its fluid contents and contraction 
of its walls. The result of these opposing forces is the extensive tearing 
of the walls of the vesicle that has taken place. The other vesicle being 
very much smaller, and not so directly in the centre of the retractive 
force, has suffered much less. 
After careful consideration of the sections and model, the conclusion 
is inevitable that the larger vesicle represents the amnio-embryonic cavity, 
and the smaller the entodermic vesicle or future yolk sac. The data on which 
this interpretation is based will be discussed in the next chapter; mean¬ 
time it may be said that it is in harmony with what we now know of the 
early primate blastocyst, and with the recent views as to the development 
of the amnion in several lower mammals. It may also be pointed out 
that this is the earliest phase of the human embryo yet observed. In 
Peters’ ovum the rudiment consisted likewise of two closed vesicles, but 
the larger or amnio-embryonic vesicle showed a differentiation of the floor 
into embryonic ectoderm, and the roof into the amnion. Our ovum 
shows a still earlier condition of the amnio-embryonic cavity, in which 
there is as yet no distinction between embryonic and amniotic ectoderm. 
