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Day 1- Zygote Stage
On the first day of fertilization, an early zygote is present. A zygote is the first diploid cell formed by the fusion of an oocyte and spermatozoa. -
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Day 2 and 3- Morula stage
On day two, the zygote forms two blastomeres that divide into the morula. The morula is a solid mass of cells. Blastomeres form a blastocyst, which is a hollow ball with an inner cavity, known as a blastula. -
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Days 4 and 5- Blastocyst hatching
On the fourth and fifth days of fertilization, the free-floating blastocyst reaches the uterine body. It is still surrounded by the zona pellucida at this point, but eventually hatches from it so it can attach to the uterine wall. -
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Day 5 to 6- Attaching blastocyst
Around day 6, the initial phase of implantation begins, called "adplantation". This requires the newly hatched blastocyst to loosely attach to the endometrial epithelium. This process requires both the blastocyst adhesion interaction with the endometrium during the "receptive window". -
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Days 7 to 12- Implantation
During this stage, the blastocyst adheres to the endometrium epithelium and implants itself through this layer into the underlying uterine lining. Trophoblast cells proliferate. Syncytiotrophoblast cells continue to dive deeper into the maternal endometrium. The maternal endometrium begins the decidual process. Extraembryonic cavities begin to form. The bilaminar disc begins to form from the inner cell mass. -
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Days 13 to 15- Extraembryonic Mesoderm and Primitive Streak
The three embryonic spaces (primitive yolk sac, chorionic and amniotic) are present. Villi surround the chorionic cavity, which are the beginning functional unit of the placenta. Outside of the villi, trophoblasts invade the maternal decidua forming maternal blood-filled lacunae. Emptying into these maternal lacuna are both uterine glands and spiral arteries held open by trophoblast cells. -
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Days 15 to 17- Gastrulation and Notochordal Process
Gastrulation continues as cells migrate from the epiblast, forming the mesoderm. The mesoderm is between the ectoderm and endoderm. A tube extends from the primitive node in the opposite direction of the primitive streak. The tube forms the axial process, then notochordal process, then finally the notochord. The notochord is key to embryonic folding. The embryonic disc folds ventrally, pinching off a portion of the yolk sac to form the lining of the GI tract. -
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Days 17 to 19- Primitive Pit and Notochordal Canal
In this stage, the neural groove and folds are first seen. The primitive pit develops from the primitive node upon cell invagination, as cells migrate and form the notochordal canal or head process. -
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Days 19 to 21- Somitogenesis
The first somites form and continue to be added in sequence caudally. They form the earliest at the cranial end of the neural groove. The three main divisions of the brain can be distinguished while the neural groove is still completely open. -
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Days 22 and 23- Heart Begins to Beat
At week 3, heart cardiogenesis begins as paired heart tubes. The heart begins to beat at day 22 and 23 and it the embryo's first formed functioning organ. Also, neural crest differentiation at the spinal cord level begins on day 22 and neural folds begin to fuse near the junction between brain and spinal cord, -
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Day 23 to 26- Rostral Neuropore Closes
Neural rostral neuropore closes within a few hours. Closure is bidirectional and it takes place from the dorsal and terminal lips and may occur in two areas simultaneously. The two lips, however, behave differently. Optic ventricle appears and the neural groove/tube space is initially filled with amniotic fluid. -
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Day 26 to 30- Caudal Neuropore Closes
The neural tube is normally completely closed. The ventricular system is now separated from amniotic fluid. Neural crest at spinal level is segregating and spinal ganglia are in series with the somites. Spinal cord ventral roots beginning to develop. -
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Day 30 to 35- Sensory Placodes
Sensory placodes, lens pit, otocyst, nasal placode, primary and secondary vesicles, fourth ventricle of brain are visible. The heart, liver, umbilical cord and mesonephric ridge are visible externally as bulges. Cranial nerves, except olfactory and optic, are identifiable in more advanced embryos. -
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Days 36 to 40
Development of the middle cerebral artery is first identified as small buds originating proximal to the anterior cerebral artery on the anterior division of the primitive internal carotid artery. Retinal pigment is present and prior to the development of the eyelids, one small sulcus or groove forms above the eye and another below it. -
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Day 41 to 47
During these days a lot develops and changes in the fetus. Olfactory nerve fibers enter the brain. The groove below and above the eye deepen, so eyelid folds develop. The enlargement of the liver stops the descent of the fetus's heart and lungs and biliary ductules develop in periportal connective tissue. Common cardiac outflow separates into aortic arch & pulmonary aorta. Throughout the embryo, bone replaces cartilage. Pancreatic hormone secretion increases in the mother. -
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Day 48 to 53
By day 48, the first generation of subsegmental bronchi is now complete. The anal membrane is defined. Sternum right and left sternal bars are present. The middle cerebral artery becomes more prominent, the plexi fuse into a single artery and further branches pierce the cerebral hemisphere.The scalp vascular plexus visible. Upper limbs begin to rotate ventrally. Future Purkinje cells develop. Lower limbs start to rotate. -
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Day 54 to 56- End of Embryonic Development
Hands and feet are turned inwards. Fingers and toes lengthen. Eyelids and external ears are visible. The head is rounded. The heart becomes prominent. In the body, the embryo experiences a straightening of the trunk, umbilical cord, intestines herniated at umbilicus. Chorionic cavity is now lost by fusion with the expanding amniotic cavity. These days mark the beginning of fetal development and the end of embryonic development.
