Embryonic cells especially active in assuring nutritional exchange between a pregnant woman and her embryo are found in the

Embryonic cells especially active in assuring nutritional exchange between a pregnant woman and her embryo are found in the trophoblast. Trophoblasts (from Greek trephein: to feed, and blastos: germinator) are cells forming the outer layer of a blastocyst, which provide nutrients to the embryo and develop into a large part of the placenta. 

The missing word from your question was trophoblast. I hope this is the answer that you were looking for.