IMBEDDING OF THE OVUM 
37 
and correspond to the allantoic vessels. The Haftstiel is, as now well 
known, a feature peculiar to the Primates, there being further no free 
allantois. 
Accurate data regarding the chronology of imbedding and development 
in the hedgehog are wanting. 
In the mouse the process of imbedding is in some respects very 
similar to the type seen in the hedgehog. The ovum becomes lodged in a 
furrow on the side of the slit-like lumen of the uterus farthest from the 
mesometrium. The epithelium disappears in its neighbourhood; decidual 
development occurs in the adjacent endometrium, which swells up into a 
thick cushion all round the ovum. The swelling gradually obliterates the 
middle region of the lumen, leaving however at first an open passage 
along the opposite wall of the uterus. The space between the lips of the 
decidual swellings is at first blocked by the ectoplacenta, but later the 
epithelium disappears—apparently as a result of atrophy due to pressure, 
the exposed decidual surfaces coalesce, and the capsule is completed. 
Besides great hyperplasia of the decidua cells, new vessels are formed 
which are directed towards the ectoplacenta, and when the union of 
trophoblast and decidua is accomplished, the placenta is developed in the 
region where the decidua capsularis is finally closed. The part of the 
decidua, therefore, which is developmentally reflexa plays the part of 
serotina. Prior to this there is generally some haemorrhage into the 
implantation cavity, and the shed blood is supposed to be used as 
pabulum for the growing ovum. 
The further development of the placenta does not concern us except 
in one particular, viz.—the uterine epithelium, as is clear from the 
work of Burckhard, plays no part in the formation of the placenta; this 
is achieved in this case, as in the case of the hedgehog, by the union 
of foetal ectoderm and decidual connective tissue, and by the opening 
of maternal blood-vessels into spaces formed in a thickened portion of the 
troph oblast. 
The time relations in the case of the mouse and rat are very accurately 
known. "They will be discussed in the chapter dealing with the age of 
the present ovum, but it may here be noted that imbedding commences 
