On the orientation of the implanting blastocyst.

In viviparous mammals the implanting blastocyst is positioned according to a number of parameters which are species specific. First, in polytocous animals the blastocysts become spaced out along the length of the uterus, secondly they take up a particular relationship to the circumference of the endometrium, and lastly the embryonic pole always assumes a constant orientation to the mesometrial-antimesometrial (vertical) axis of the uterus. The first two factors have been reviewed by Blandau (1961) and are generally agreed to be uterine-dependent. Wilson (1963) has shown that even melanoma cells introduced into the uterine lumen of the pseudopregnant mouse become segmented into clumps along the length of the uterus and are always positioned in the antimesometrial segment of the lumen as are blastocysts during normal development. This paper is limited to discussing the third parameter only—the orientation of the inner cell mass of the blastocyst with regard to the underlying maternal tissue. The evidence is taken from studies of implantation in the mouse in which the normal blastocyst invariably implants with its inner cell mass directed towards the mesenteric attachment of the uterus.