Blasto!

Early Blastocyst

0.1 - 0.2 mm

4 days post-ovulation

About four days after fertilization, the morula enters the uterine cavity. Cell division continues, and a cavity known as a blastocele forms in the center of the morula. Cells flatten and compact on the inside of the cavity while the zona pellucida remains the same size. With the appearance of the cavity in the center, the entire structure is now called a blastocyst.
The presence of the blastocyst indicates that two cell types are forming: the embryoblast (inner cell mass on the inside of the blastocele), and the trophoblast (the cells on the outside of the blastocele).

http://www.visembryo.com/baby/stage3.html