50 
AN EARLY HUMAN OVUM 
known in the case of the white mouse, but they have also been ascertained 
with considerable exactness in the cases of the guinea-pig, rabbit, pig and 
doer. The nature of the attachment of the ovum to the uterus and its 
o 
course of development are so different in the pig and dog from what 
they are in man, that comparison is impossible, except with regard to 
one point which will be taken later. The relatively short gestation 
period of the three rodents, and the individuality in the behaviour of the 
ova of different species, compel us to exercise caution in applying the 
facts of their development to the elucidation of human embryology. 
In the mouse the rate of development is known very accurately up 
to the 10th day, and the results of Sobotta, Burckhard, Jenkinson, and 
Melissinos show no great difference in respect of chronology. Fertilization 
occurs very soon after coitus; by the end of the first 12 hours the ova 
are for the most part in the two-celled stage of segmentation; and by the 
end of 48 hours they consist of from 1G to 24 blastomeres. Develop¬ 
ment then proceeds more rapidly, and the differentiation into an outer 
layer and an inner group of cells becomes apparent. The zona pellucida is 
still complete. By the end of the 3rd day the ova have reached the uterus, 
the zona pellucida is beginning to disappear, and the morula has become 
a hollow blastocyst. The ovum now elongates, and up to the beginning 
of the 5th day is engaged in the process of inversion of the germinal layers. 
On the 4th day implantation commences by the removal of the uterine 
epithelium in the neighbourhood of the ovum, and fixation occurs on the 
6th day at the earliest, though not until the 8th day does the ecto- 
placenta take form. At this point, however, the lagging behind of the 
embryo relatively to the trophoblast in the human ovum produces a state 
of affairs so different that further comparison becomes impossible. 
Duval indicates the 7th day as the period of attachment in the rabbit, 
but the ovum in this case is not completely imbedded, and the develop¬ 
ment of the blastocyst is quite different from that in the human subject. 
On the other hand, the similarity of the process of imbedding in the 
guinea-pig to what is theoretically probable in the human subject makes 
it a more favourable subject for comparison. 
Yon Spee figures numerous ova of the guinea-pig either free in the uterus 
