Day 8:

1. 1. Blastocyst is partially embedded in the endometrial stroma.

2. 2. Trophoblast has differentiated into the inner cytotrophoblast and outer syncytiotrophoblast.

3. 3. Formation of the bilaminar embryonic disc: differentiation of the inner cell mass into the hypoblast layer adjacent to the blastocyst cavity and epiblast layers adjacent to the amniotic cavity.

Day 9:

1. 1. Blastocyst is completely embedded in the endometrium and the surface defect is closed by fibrin coagulum.

2. 2. Exuberant growth of trophoblast occurs at the embryonic pole.

3. 3. Large lacunae form in the syncytiotrophoblast (lacunar stage).

4. 4. Primitive yolk sac is formed.

Days 11 and 12

1. 1. Blastocyst is completely embedded in the endometrium and the defect is closed by surface epithelium.

2. 2. At the embryonic pole, the syncytiotrophoblast erodes the epithelial lining of the maternal capillaries (sinusoids).

3. 3. Maternal blood enters the lacunar system, leading to formation of the uteroplacental circulation.

4. 4. There is formation of extraembryonic somatic mesoderm in between the cytotrophoblast and the amnion.

5. 5. There is formation of extraembryonic splanchnic mesoderm in between the cytotrophoblast and the yolk sac.

6. 6. The extraembryonic mesoderm develops from yolk sac cells.

Day 13:

1. 1. Primary villi are formed (cellular columns of cytotrophoblast surrounded by syncytiotrophoblast).

2. 2. Secondary or definitive yolk sac develops.

3. 3. There is formation of the chorionic plate from the extraembryonic mesoderm, which lines the inner aspect of the trophoblast.

4. 4. The connecting stalk develops, which is the future umbilical cord.

Gastrulation is the process of formation of the trilaminar germ disc, giving rise to three layered embryonic discs (ectoderm, mesoderm and endoderm).

Gastrulation begins with the formation of the primitive streak, which occurs at around the fifteenth to sixteenth day.

The cephalic end of the primitive streak is known as the primitive node, which is a slightly elevated area surrounding the small primitive pit.

Cells of epiblast (ectoderm) migrate towards the primitive streak and invaginate between the bilaminar embryonic disc (between the epiblast and hypoblast), giving rise to mesoderm.

The epiblast develops into the ectoderm and the hypoblast develops into the endoderm.

The mesoderm is absent in two locations where the original two germ layers stay together, which are as follows:

1. 1. Prechordal plate at the cephalad end on the disc, which is soon replaced by buccopharyngeal membrane.

2. 2. Cloacal plate at the caudal end, which is replaced by cloacal membrane in future.