SIZE: 0.2 mm

TIME PERIOD: 13 days post-ovulation

Placenta Formation
Chorionic villi "fingers" form in the "placenta" anchoring the embryo to the uterus. Blood vessels begin appearing first in the "placenta" surrounding the embryo. The yolk sac begins producing hematopoietic or non-nucleated blood cells.

Stalk Formation
By the end of stage 6a, the embryo is attached to the developing placenta by a stalk later to become part of the umbilical cord.

Gastrulation

In Carnegie Stage 1, the exit of the first polar body through the zona pellucida determined the gastrulation axis.

Stage 6b begins as a narrow line of cells appear on the surface of the formerly 2 layered embryonic disc. This "primitive streak" marks bilateral symetry in the embryo and indicates gastrulation has begun - the migration of cells from the outer edges of the disc into the primitive streak and down creating a new third layer. The embryo now has endoderm, mesoderm and ectoderm layers.

ECTODERM Top cell layer of the embryonic disc. Will later form: skin, hair, lenses of the eyes, lining of the internal and external ear, nose, sinues, mouth, anus, tooth enamel, pituitary and mammary glands, and all parts of the nervous system.

MESODERM Middle cell layer of the embryonic disc and precursor to the muscles, bones, lymphatic tissue, spleen, blood cells, heart, lungs, reproductive and excretory systems.

ENDODERM Inner cell layer of the embryonic disc from which will form the lining of the lungs, the tongue, tonsils, urethra and associated glands, bladder and digestive tract.

Appearance of  gene NOTCH 1