The lacunae of the trophoblast begin to communicate with each other. At some places, the cells of the syncytiotrophoblast have grown so far that they dissolve the endothelial lining of maternal sinusoids. Maternal blood can thus enter the lacunar system, thereby establishing connections between the arterial and venous branches of the maternal circulation.
The trophoblast and parietal hypoblast (synonyms: extra-embryonic hypoblast; later extra-embryonic endoderm; Heuser membrane) grow at different rates. Due to this difference in growth clefts appear between the trophoblast and the parietal hypoblast. Within these clefts, cells known as extra-embryonic mesoblast (synonyms: extra-embryonic reticulum, magma reticulare, S5c) can be seen. The origin of the extra-embryonic mesoblast cells is not yet clear. Potential sources are the caudal region of the epiblast, at a time before the primitive streak is formed, and the parietal hypoblast.