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The human zygote, the first cell formed following fertilization. In the middle, the 2 pronuclei (one maternal and paternal) containing the parents genetic material. These will fuse to form the new genetic mix of the child.
The blastocyst, the cell mass formed by cell divisions in the first two weeks following fertilization. This image, at the end of the first week, shows the blastocyst (right) "hatching" out of the surrounding protective shell, the zona pellucida (left).
A summary of the first 2 weeks of human development. The image shows from ovulation to fertilization and through to implantation during the second week.
After implantation the developing cells release a hormone to support the pregnancy. Pregnancy tests detect this hormone in the mother's urine.
The embryo in the third week is initially a flat disc of cells. This embryo is 15 - 17 days old and 0.4 mm in diameter, the head will develop at the top and the placenta at the bottom.
This embryo in the fourth week (about 21 days) is now folded and 2 mm in length. The head will develop at the top and the placenta at the bottom. The bulge visible on the left is the early heart, the first organ to form. The valley or groove in the middle will form the early spinal cord and brain.
Dr Mark Hill is a senior lecturer in the School of Medical Sciences, The University of New South Wales, Sydney Australia.
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I have been an educator to both Medical and Science students in Embryology and Cell Biology at the University level for the last 13 years. I am interested in how new technologies can be used to best present, and help aid the understanding of, complex developmental processes.
My laboratory research interests are looking at the relationship between cell shape and function, in particular in neurological development and disease.