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Day 1- Secretory Phase (Stage 1)
During stage one of carnegie, the human zygote has not been formed yet into a single zygote nucleus. The zona pellucida forms a thick extracellular layer around both the egg and the developing embryo to be. -
Day 2- Secretory Phase (Stage 2)
During stage two, first cell mitotic divisions of the zygote form 2 blastomeres that continue to divide to form the morula. -
Day 4- Secretory Phase (Stage 3)
During stage three, the blastocyst has reached the uterine body that is still enclosed in the zone pellucida and it hatches from the surrounding extracellular matrix. After it hatches, the blastocyst can attach and implant into the uterine wall. -
Day 7- Secretory Phase (Stage 5)
During stage five, implantation is completed. The trophoblast cells proliferate and the maturnal endometrium begins the decidual process and the endometrial stroma accumulates fluid. The extraembryonic cavities also begin to form and the bilaminar embryonic disc forms from the inner cell mass. -
Day 16- Secretory Phase (Stage 7)
During stage 7, gastrulation continues as cells migrate from the epiblast to continue forming the mesoderm. A tube extends under the ectoderm in the opposite direction to the primitive streak. The notochord is key to embryonic folding and regulation of ectoderm and mesoderm differentiation. -
Day 20- Secretory Phase (Stage 9)
Stage nine occurs during week three between 19 to 21 days. At this point, the embryo is about 1.5 to 2.5 mm in size. -
Day 22- Secretory Phase (Stage 10)
During stage ten, there are changes in the ectoderm and the mesoderm. In the ectoderm, neural fold deepens, edges approach midline, and the neural fold fuses. In the mesoderm, there is the continuation of segmentation of paraxial mesoderm. -
Day 32- Secretory Phase (Stage 14)
During stage 14, there are changes in the ectoderm, mesoderm, the head, the body, and the limbs. In the head, the 1st, 2nd, and 3rd pharyngeal arches are seen, along with the forebrain and the site of lens placode. In the body, the heart, liver, umbilical cord, and the mesonephric ridge is formed. The upper and lower limb buds are also formed. -
Day 41- Secretory Phase (Stage 17)
During stage 17, the nasal pits moved ventrically and there are new features being formed such as the auricular hillocks in the head, and hand digital rays for limbs. -
Day 50- Secretory Phase (Stage 20)
During the 20th stage, the zygote has more of a baby appearance. One new feature during this stage is hearing. -
Day 56- Secretory Phase (Stage 23)
This is the final stage of the carnegie stage. After this stage, the embryo is considered a fetus. The embryo now has eyelids, external ears, and a rounded head. As for the body, the trunk is now straightned and the hands/ feet are turned inward.
