IMBEDDING OF THE OVUM 
39 
pig the endometrium does not show the slightest trace of alteration from 
the non-pregnant state until the epithelium has disappeared, and the ovum 
commences to attack the deeper tissues; decidual development then rapidly 
follows. Whether this is the same in the hedgehog is not quite clear. In 
all these cases the ovum becomes completely enclosed, and after a period 
in which it lies free in the implantation cavity, it obtains attachment to 
the maternal connective tissue by means of the development of a special 
part of its ectoderm. 
While the present ovum is the youngest yet described it is not yet 
sufficiently early to demonstrate the actual mode of implantation in man, but 
it brings us nearer to a comprehension of the process in several respects. 
In regard to the antecedent phenomena there is no reason for 
believing that they are different from those in other forms, and we 
may consider that the ovum reaches the uterus as a small blastocyst 
still enclosed in the zona pellucida. The cavity of the human uterus 
is of course vastly larger, relatively to the ovum, than that of the 
hedgehog, mouse or guinea-pig, but it is narrow and slit-like. It 
differs from that of the mouse in having, as far as the corpus uteri is 
concerned, a perfectly smooth surface, free from crypts or furrows, with the 
exception of the mouths of the glands, which are too minute in an undilated 
state, to admit an ovum. The idea that the ovum becomes imbedded in 
a gland is apparently not now held by any one. Our ovum is imbedded 
near the centre of a smooth, rounded elevation, similar to, but higher than, 
many other small areas lying between the furrows, and it appears highly 
probable that the elevations and furrows alike have arisen as the result 
of decidual proliferation. We have no evidence that this development 
takes place prior to the imbedding of the ovum. Indeed, from what 
occurs in the mouse and guinea-pig, we might from analogy infer that 
the human ovum may imbed itself in practically unaltered endometrium. 
This question will be considered again in the chapter on the age of the 
ovum, and the relations of imbedding to menstruation. 
When we consider the appearances of this elevation, the shape of the 
uterine lumen and the characters of the mouth of the implantation cavity 
in our specimen, we must conclude that in the initial stages the process 
