File talk:Stage5 bf01.jpg

Carnegie No. 7700 (figs. 18-22). Described by Hertig and Rock (1941). Hysterectomy. Posterior wall of uterus. Chorion, 0.948 x 0.835 mm. Chorionic cavity, 0.55 x 0.498 mm. Embryonic disc, 0.204 x 0.165 mm. Presumed age, 12 days.

The space outlined by the internal surface of the chorion now appears distended again. However, a gradient of differentiating trophoblast from abembryonic to embryonic pole is still evident. Intercommunicating lacunae are visible in the syncytiotropho- blast. The primary umbilical vesicle, surrounded by a meshwork, can be seen. The endometrial stroma is edematous and decidua is developing. Section 6-1-5

The bilaminar embryonic disc of No. 7700, stage 5c. The amnion overlying the disc does not seem to be complete. The epiblast is a pseudostratified columnar epithelium. The sur- rounding cytotrophoblast is evident, and lacunae can be seen in the syncytiotrophoblast. Cytotrophoblastic clumps may be regarded as an indication of the future chorionic villi. Section 6-1-5.