The accompanying figure illustrates the process of human development from fertilization to implantation. Describe what happens from day 1 through days 6-7 of this process

Question

What will be an ideal response?

Answer 1

Students should outline how a blastocyst forms into an early embryo, and implants itself in the endometrium. They may include some of the following steps.

DAYS 1–2 . The first cleavage furrow extends between the two polar bodies. Later cleavage so cells become asymmetrically arranged. They are loosely organized with space between them.
DAY 3 . After the third cleavage, cells abruptly huddle into a compacted ball, and tight junctions among the outer cells stabilize.
Gap junctions formed along the interior cells enhance intercellular communication.
DAY 4 . By 96 hours, the embryo is a solid ball of cells called a morula. Cells of the surface layer will function in implantation and give rise to a membrane, the chorion.
DAY 5 . A fluid-filled cavity called the blastocoel forms in the morula and the inner cell mass forms. Differentiation occurs in the inner cell mass and gives rise to the embryo proper. This embryonic stage is the blastocyst.
DAYS 6–7
. Some of the blastocyst's surface cells attach themselves to the endometrium and start to burrow into it. Implantation has started.