28 
AN EARLY HUMAN OVUM 
into a parietal and a visceral layer; it does not form a distinct lamella 
round the wall of the cavity; and there are no protrusions from it 
representing future mesoblastic villi. 
(5) The embryonic rudiment is represented by two eccentrically placed 
vesicles slung in the mesenchyme by fine protoplasmic threads. They 
are quite separated from the wall of the blastocyst by mesenchyme, 
and the cells forming the two sacs have definite and different characters, 
but inter se show no differentiation. The cells of the larger (amnio- 
embryonic) vesicle are cubical; those of the smaller (entodermic) vesicle 
are flattened. 
Remarkable as some of the features of this new ovum are, there is 
no reason to suppose that it is in any way abnormal or pathological. 
Every one of its characters, as we shall now proceed to show, harmonises 
admirably with known later stages, it is in no sense contradictory or 
bizarre. It is not only consistent in itself, but is also consistent both 
with admitted facts and with inferences founded on these facts. 
In the following discussion we shall first establish the position of our 
ovum relative to the earliest ova hitherto recorded, more especially those 
of Leopold (1906) and of Peters, and consolidate the basis of our inter¬ 
pretation both of the trophoblast and of the embryonic rudiment. 
COMPARISON OF THE PRESENT OVUM WITH THOSE OF LEOPOLD 
AND OF PETERS. 
In Leopold’s ovum nothing like an embryonic rudiment was found. 
Towards the centre of the implantation cavity is a sac which Leopold 
regarded as the blastocyst. It is a thin walled and irregular vesicle 
containing tissue similar to the mesenchyme in the present ovum and in 
that of Peters, but infiltrated with red-blood corpuscles, obviously maternal, 
and separated more or less from the wall of the vesicle by a space contain¬ 
ing blood corpuscles (cf. Leopold, 1906, Plate x, Fig. 18). The wall of the 
blastocyst is described as a thin mantle of ectoderm showing two layers of 
cells, the inner consisting of rounded or oval elements closely applied to 
one another and almost filled by large darkly staining nuclei, the outer 
