The interhemal membrane of the chorioallantoic placenta in the insectivoreSuncus murinuswas investigated by means of electron microscopy. In the interhemal membrane the syncytiotrophoblast clearly intervened between the hypertrophied maternal endothelium and the fetal endothelium by day 20 of pregnancy. Although the syncytiotrophoblast showed a sieve-like feature from day 20 to 24, it was distinctly continuous. The syncytiotrophoblast, however, became discontinuous in most areas of the labyrinthine zone after day 24, and finally both projections of the maternal and the fetal endothelium contacted each other. These findings indicate the focal existence of an endothelio-endothelial condition within an otherwise endothelio-chorial placenta.
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