Cluster of cells formed after the first 4 days after fertilization that enters uterus

At 5-6 days: fluid formed cavity in the ball of cells

1. At 5 days: after formation of morula, but before implantation

2. Cluster of cells with a fluid filled cavity (blastocele)

3. Possesses embryoblast and trophoblast

1. Outer shell of cells in blastocyst that develops into placenta

2. Mediates the implantation of the embryo into the endometrium, but are never incorporated into mother's body or fetus

Cluster of cells adhering to one pole of the inner side of the trophoblast, within the blastocyst, that forms the embryo

At 7-8 days: flat, two-cell (epiblast and hypoblast) layered disk formed by embryoblast

1. Thicker layer of bilaminar disk that forms the floor of the amniotic cavity

2. The cells that form a dome over the epiblast eventually become the amnion

Second layer of bilaminar disk ;

The dome of cells that forms below the hypoblast turn into the primary yolk sac

Embryo is in the shape of 2 fluid-filled bubbles; where the bubbles adjoin is where the bilaminar disk is situated ("double bubbles")

The space between the embryo and the trophoblastic shell;

By the time that the two sacs have formed, more fluid has accumulated between the developing embryo and trophoblastic shell; space gets larger and larger until its overall diamter dwarfs the embryo

1. Tether formed by the loose connective tissue that surrounds the embryo and lines the inside of the trophoblastic shell;

2. Keeps embryo tethered inside the coelom;

3. Beginning of the umbillical cord

1. At 15 days:  A midline longitudinal groove at the caudal end of the bilaminar disk formed by a thickened linear band of epiblast;

2. Site of proliferation and downward migration of epiblast cells into the region between epiblast and hypoblast, and into hypoblast layer itself;

3. Forms 3 cell layers;

4. Disappears after 26 days

1. The formation of 3 germ layers: ectoderm, mesoderm, and endoderm

by the primitive streak

2. Bilaminar disk now trilaminar disk

3. Ectoderm will transition into overlying amniotic sac and endoderm will transition into underlying yolk sac

The cranial end of the primitive streak where the notochord extends from (extends forward from primitive streak, but beneath surface of embryonic disk)

Rod-shaped structure formed from the primitive node that is bounded by ectoderm above, mesoderm on each side, and endoderm beneath it; 

Forms the primitive backbone that establishes the longitudinal axis of the body;

Induces development of neural groove (forms brain and spinal cord)

1. Formed during gastrulation

2. Only ectoderm and mesoderm- No intervening mesoderm

3. A surface depression at the cranial end of the embryonic disk that becomes the mouth

1. Formed during gastrulation

2. Only ectoderm and endoderm- No intervening mesoderm;

3. A second surface depression at the caudal end of the embryonic disk that forms the anus

1. At day 21: 1st step of neurulation

2. Upwellings/plateaus of ectoderm at cranial end, induced by notochord

1. Two longitudinal ridges on either side of the midline that extend caudally  from neural plate and grow towards each other to fuse and form the early neural tube

2. Give rise to sensory ganglia of spinal and cranial nerves

1. Formed by the fused neural crests that will ultimately form brain and spinal cord (forms around neural groove)

1.The process of forming the neural tube (which will become brain and spinal cord);

2. Cranial end (cranial neuropore) closes at 24 days and caudal neuropore closes at 26 days

1. Occurs soon after onset of gastrulation (~15 days);

2. Mesoderm differentiated into four components: paraxial mesoderm, intermediate mesoderm, somatic lateral plate mesoderm, and splanchnic lateral plate mesoderm

1. The portion immediately lateral to neural tube that forms somites (pairs of cubes or blocks of tissue) situated on either side of neural tube;

2. Ultimately forms axial skeleton and associated musculature, and adjacent dermis of the skin

1. Paired and segmented mesoderm located lateral to somites;

2. Ultimately forms part of urogenital system (kidneys and gonads)

1. A plate underlying the ectoderm located laterally to intermediate mesoderm

2. Ultimately forms body wall

1. Plate of mesoderm that surrounds the yolk sac, located laterally to intermediate mesoderm

2. Ultimately forms circulatory system and future gut wall

1. After day 20: cranial end and caudal end move ventrally to approach each other to help form amnionic sac

2. Completed by about 30 days

1. After day 20: Right and left sides of the embryo fold ventrally to approach each other to help form amnionic sac

2. The embryonic disk arches up and herniates into the amnionic sac and the yolk sac is partially incorporated into the inside of the embryo

3. Process completed by about 30 days