EDELMANN / SCIENCE PHOTO LIBRARY. EDELMANN / SCIENCE PHOTO LIBRARY.
The human embryo at around 4 weeks, showing the umbilicus & blood vessels of the placenta (top left) , which connect the embryo with its mother's circulation. At this time most of the important organ systems have been formed. The front end of the neural tube has closed & the primitive brain starts forming. The bulges on either side of the brain are optic vesicles, the first development of the eyes. The heart has formed during the 3rd week by the union of 2 blood vessels underneath the head, which elongate & bend into an S-shaped, & finally into an incomplete 4-chambered structure.
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