INTRODUCTION 
3 
the ova, such for instance as that described by Reichert, showed a small 
area of the decidua capsularis (formerly called reflexa) over the blasto¬ 
cyst, which was of a different nature from the rest of the capsule, and 
apparently composed of cicatricial tissue. They were, notwithstanding, 
completely enclosed by organised tissue. In Peters’ ovum, however, and 
also in one described by Graf v. Spee (1905) there is a relatively large 
area from which decidua is absent, and its place is occupied by a mass of 
fibrin and blood-clot (the “ Gewebspilz ”). The aperture in the wall of the 
implantation cavity occupied by this mass was considered by Peters, and 
also by Graf v. Spee, as the point of entrance of the ovum into the substance 
of the mucosa, but their preparations do not b} 7 themselves conclusively 
demonstrate the actual process by which the ovum is implanted. To 
prove liow this is effected still earlier stages are required. Our young 
ovum is a further step in the direction of assured knowledge, and as w T ill 
be seen later necessitates some modification in the interpretation of the 
“ Gewebspilz ” completing the capsule in Peters’ specimen, while our 
ovarian ovum, which is the youngest hitherto described implanted in the 
ovary, throws considerable light on the nature of the imbedding process. 
In the absence of the early stages in * the human subject it is 
necessary to make use, for the purposes of interpretation, of the data 
provided by Comparative Embryology, but the remarkable variability in 
the methods of implantation and in the details of placentation in the 
different mammalian orders, speaks for a certain specific character of the 
embryological processes involved. Caution, therefore, is required in grafting 
any data derived from the investigation of the conditions in lower mammals 
on to the facts known for the human ovum, and the more so as the young 
ovum we have to describe accentuates the very special features of the 
human blastocyst in its early phases. 
The only competent analogy with the higher primate ovum among 
the lower mammals is to be found among the forms in which there is 
likewise a decidua capsularis, for instance the hedgehog among the 
insectivora, and the mice, rats, and guinea-pig among the rodents. It is 
to be noted that in these forms, as in the Primates, the amnion is closed 
from the first, and that the blastoderm shows the phenomenon, to a greater 
