COMPARATIVE DATA 
51 
or in process of imbedding. None of these are earlier than the beginning 
of the 7th day after coitus, and nearly all are in the early blastocyst 
stage. There is no marked change up to the end of the 7th day. At 
7 days 13 \ hours the ovum is a hollow vesicle of considerable size, 
completely imbedded, and shows an enormous advance compared with ova 
24 hours younger. This may be explained by supposing that whereas 
the growth of the ovum is relatively slight before implantation, develop¬ 
ment proceeds very quickly after imbedding is complete. 
It seems improbable that the human ovum could be imbedded 
earlier than that of the guinea-pig, and we therefore agree with the 
opinion expressed by Graf von Spee that imbedding in the human 
subject cannot take place sooner than 7 days after fertilization. By this 
time in all probability segmentation is complete, but the ovum has hardly 
increased in size, still retaining the dimensions of the unfertilized oocyte, 
i.e. about '2 of a millimetre in diameter. 
The rate of development after imbedding is more rapid in the rabbit 
than in the mouse, and much more rapid than in the guinea-pig. These 
variations are probably, in part at any rate, to be explained by the 
different methods of implantation. Development seems to be retarded 
in the early stages in forms with a decidua capsularis, and in the human 
ovum the retardation of the evolution of the embryonic rudiment compared 
to the trophoblast is a very striking feature. If, therefore, seven days 
be arbitrarily allowed for segmentation and the early phases of implantation, 
another seven days does not seem too much to allow for the growth and 
development of the ovum up to the stage presented by our case, when 
we consider the rate of growth in the mouse and guinea-pig, and allow 
for the much longer gestation period in the human subject. 
In comparing the present ovum with other human ova, it seems 
advisable, in order to restrict the range of error as far as possible, to 
consider only those which may be estimated as at most a week older than 
it is; therefore no ovum larger than Graf v Spee’s “ Gle ” is included in 
our series. Further, only those are considered which have a reasonably 
complete and precise history, and have been obtained under favourable 
circumstances, or in such condition as precludes retention in utero for 
