BIOLOGY OF EGGS AND IMPLANTATION 



855 



20 



TUBAL 

 END 



7 



'a 



40 60 80 



OF UTERINE LENGTH 



100 



Fig. 14.18. Positions of rabbit blastocysts (dots) in utero (bars) from the 3rd to the 8th 

 day post coitum. There is little change in position during days 3 and 4. Even distribution is 

 achieved 6 to 7 days -post coitum. The crosses in the 8-day uterine horn represent the position 

 of blastocyst models which had been in the uterus for 2 days (Boving, 1954). 



and to-and-fro motion approximately every 

 30 seconds. This seems to be effected by a 

 change in the tone of the muscles forming 

 the uterine dome. The rotational motion 

 could provide an orientational mechanism, 

 because eventually all surfaces of the blas- 

 tocyst would come in contact with the dome. 

 In the in vivo observations, it seemed that 

 the blastocyst is "grasped" by the muscular 

 action of the uterus, and by the 7th day 

 post coitum is confined along the antimeso- 

 metrial border. 



As implied earlier, the orientation of the 

 blastocyst with reference to the uterus and 

 mesometrium varies considerably in differ- 

 ent species. It may be mesometrial as in the 

 Pteropodidae and Tarsiidae, antimesome- 

 trial as in most rodents and insectivores, or 

 orthomesometrial as in the Centetes and 

 Hemicentetes (Mossman, 1937). 



The orientation of the embryonic disk 

 within the uterus is remarkably constant in 

 closely related species but varies greatly 

 in different orders. Thus the inner cell 

 mass at the time of attachment may be di- 

 rected toward the mesometrium in the 

 rodents, toward the antimesometrial side in 

 the vesperilionid bats and some insectivores, 

 or toward the lateral side as in the golden 

 mole. With the possible exception of the 



rabbit and guinea pig, the role of the blas- 

 tocyst in determining the pole of attachment 

 is unknown. 



Alden (1945) reversed the mesometrial- 

 antimesometrial axis of the uterus of the 

 rat by surgical means and demonstrated 

 that, regardless of the position of the altered 

 segment, the implanting embryos were cor- 

 rectly oriented relative to the uterus. Ap- 

 parently, gravity alone is not of great 

 importance in determining the pole of at- 

 tachment, at least not for the rat egg. 



Before the cells of the trophoblast can 

 come into contact with the uterine epi- 

 thelium, either the tough and resistant zona 

 pellucida must be removed or the cells of 

 the living trophoblast must penetrate the 

 zona. A number of investigators have 

 thought chiefly in terms of the removal of 

 the mucous coat and zona pellucida by 

 uterine factors. As we will see, others have 

 been impressed by the possibility of par- 

 ticipation by the trophoblast. 



In 1935 Hall presented evidence which 

 seemed to support the former view. He 

 found that in rats and mice the zonae 

 pellucidae disappear rapidly when im- 

 mersed in fluids of pH 3.7 or below. In 

 less acid solutions (pH 4 to 5), they were 

 affected much more slowly. Acidified Ring- 



