ATTACHMENT OF MAMMALIAN EMBRYO TO UTERUS. 173 



On the Causes which lead to the Attachment 

 of the Mammalian Embryo to the Walls of 

 the Uterus. 



By 



Richard Assheton, M.A. 



With Plate 19. 



I. 



The First Attachment of the Rabbit Embryo to the 

 Walls of the Uterus. 



By the end of the eighth day, if not actually attached to the 

 walls of the uterus, the embryos have become definitely located, 

 and their presence is made evident from the exterior of the 

 uterus by a bulging in the wall opposite the mesometrium. 

 Frequently by this age they are actually attached to the uterus, 

 and cannot easily be extracted without damage. 



It must also be noted that the embryos by this time no longer 

 lie anyhow in the uterus, but when definitely located the position 

 of the embryo to the uterus is such that the embryonic area 

 is always towards the mesometric wall of the uterus. 



This is a very important fact. It is probably necessary for 

 the development of the rabbit that it should be thus situated. 

 It would be very awkward if the embryo became fixed in any 

 other position. The shape of the uterus at this stage, and the 

 shape of the blastodermic vesicle at this stage, are so beauti- 

 fully adapted one to the other as to render any other position 

 almost impossible. The blastodermic vesicle by this time is a 

 slightly elongated body whose lower pole is semicircular in 

 transverse section, while the upper pole is much flattened. If 



