-
Day 1: Stage 1 (Secretory Phase)
Fertilization occurs and the zygote begins to form -
Days 2-3: Stage 2
The morula and blastocyst form -
Days 4-5: Stage 3
The blastocyst hatches and is now free-floating. The zona pellucida is lost -
Day 6: Stage 4
Adplantation, the initial step of implantation, occurs -
Days 7-12: Stage 5
This is when implantation of the zygote into the uterine wall occurs -
Days 13-15: Stage 6
This is when the chorionic cavity forms -
Days 16-17: Stage 7
Gastrulation begins. The ectoderm, mesoderm, and endoderm form -
Days 18-19: Stage 8
Neural neurogenesis begins and the neural groove and folds are visible -
Days 20-21: Stage 9
Musculoskeletal somitogenesis begins and the first somites form and continue to be added in sequence. The neural crest is visible. Heart cardiogenesis begins -
Days 22-23: Stage 10
The neural crest differentiates at the spinal cord level. The neural folds begin to fuse near the junction between the brain and spinal cord. The heart begins to beat and becomes the first functioning embryonic organ -
Day 24: Stage 11
The thyroid begins to form. Neural rostral neuropore closes for a few hours. The ventricular optic nerve appears and the neural groove is initially filled with amniotic fluid -
Days 25-27: Stage 12
The pituitary gland forms. The liver begins its development. -
Days 28-31: Stage 13
The neural tube is completely closed and the amniotic fluid is separated from the ventricular system. The respiratory system continues to grow and the ability to hear develops -
Day 32: Stage 14
The upper and lower limb buds start to form. There is the first appearance of the future cerebral hemispheres. -
Days 33-36: Stage 15
Cranial nerves are now identifiable. The heart and lungs descend into the thorax -
Days 37-40: Stage 16
The eyelids start to form and the head and lips develop further. The limb buds grow and enter the hand plate -
Days 41-43: Stage 17
Smell olfactory nerves enter the brain. The heart separates and forms the aortic arch and pulmonary aorta -
Days 44-47: Stage 18
Limb bones are formed. The female uterus begins to form if the fetus is female. The liver continues to form and the bile ducts are developed -
Days 48-49: Stage 19
Eyelid folds form the eyelids. The bronchial trees in the respiratory system reconstruct. The ovaries, testes, and uterus develop. -
Days 50-51: Stage 20
The upper limbs begin to rotate ventrally. The cerebellum cell layer and the first Purkinje fibers form. -
Days 52-53: Stage 21
The anterior communicating artery forms. The upper and lower limbs rotate. -
Days 54-55: Stage 22
Fingers and toes lengthen. hCG stimulates testosterone production. The tongue and taste buds begin to develop -
Day 56: Stage 23
This is now the end of the organogenesis period. The fetus now had a nose, eyes, eyelids, external ears, and a rounded head. Upper limbs are longer and are bent at the elbow.
