38 
AN EARLY HUMAN OVUM 
on the fifth day after fertilization, and that the ovum at that period is a 
small hollow blastocyst, not materially larger than the fertilized ovum 
before the commencement of segmentation. 
In the guinea-pig imbedding seems to occur in a different fashion. 
According to the researches of Graf v. Spee the ovum, while still in a 
very early blastocyst phase, loses its zona pellucida, and actively attacks 
the endometrium. It destroys a small area of the epithelium, and passing 
through the gap so produced, continues its destructive action on the 
connective tissue of the mucous membrane until a cavity is formed 
sufficient for its complete inclosure. The opening in the epithelium is 
blocked for a time by the cells of the future ectoplacental pole of the 
ovum, the epithelial and foetal cells remaining in contact at this point. 
Necrosis, followed by solution of the dead cells, now overtakes a 
considerable area of the endometrium, with the result that a considerable 
space ( Implantationshof ) is formed round the ovum. This is filled 
with fluid and is lined by more or less necrotic and dissolving tissue. 
Outside this zone, active proliferation takes place leading to the forma¬ 
tion of a decidua which completely surrounds the blastocyst. The placenta 
is formed from the ectoplacenta, which becomes drawn down, owing to 
changes in the uterus, towards the middle of the lumen ; maternal blood¬ 
vessels are developed in this position, and the final result is that, in spite of 
the initial difference in the manner of implantation, the relations of placenta 
to uterus are very similar to those in the mouse. It is sufficiently clear 
that in this case also the uterine epithelium takes no part in the formation 
of the placental tissues. 
The imbedding of the guinea-pig ovum is effected on the seventh day 
after coitus; segmentation is complete, but the size of the ovum is not 
appreciably increased. When implantation has taken place the ovum 
grows and develops with great rapidity. 
In all these cases fertilization apparently occurs in the upper part of 
the Fallopian tube; segmentation is completed during the transit of the 
ovum to the uterus, which it reaches but little increased in size and still 
enclosed in the zona pellucida. This envelope now rapidly disappears, and 
implantation of the ovum takes place forthwith. In the mouse and guinea- 
