XI.] ATTACHMENT OF THE OVUM. 347 



envelope. It soon fuses with the zona radiata, or at 

 any rate the zona ceases to be distinguishable. 



While the above changes have been taking place 

 the whole blastodermic vesicle, still enclosed in the 

 zona, has become attached to the walls of the uterus. 

 In the case of the typical uterus with two tubular 

 horns, the position of each embryo, when there are 

 several, is marked by a swelling in the walls of the 

 uterus, preparatory to the changes in the wall which 

 take place on the formation of the placenta. In the 

 region of each swelling the zona around the blasto- 

 dermic vesicle is closely embraced ia a ring-like fashion 

 by the epithelium of the uterine wall. The whole 

 vesicle assumes an oval form, and it lies in the uterus 

 with its two ends free. The embryonic area is placed 

 close to the mesometric attachment of the uterus. In 

 many cases peculiar processes or villi grow out from 

 the ovum (Fig. 114, 4, sz) which fit into the folds of 

 the uterine epithelium, The nature of these processes 

 requires further elucidation, but in some instances 

 they appear to proceed from the zona (rabbit) and in 

 other instances from the subzonal membrane (dog). 

 In any case the attachment between the blastodermic 

 vesicle and the uterine wall becomes so close at the 

 time when the body of the embryo is first formed out 

 of the embryonic area, that it is hardly possible to 

 separate them without laceration ; and at this period — 

 from the 8th to the 9th day in the rabbit — it requires 

 the greatest care to remove the ovum from the uterus 

 without injury. It will be understood of course that 

 the attachment above described is at first purely super- 

 ficial and not vascular. 



