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Monitoring Stem Cell Hisiahch 
Figure 101: Neumlation and Notochordal Process 
(www.viseinbryo.com by Mouseworks, Inc.) 
The mesoderm still adjacent to the neural tube resolves into the 
form of paired blocks on either side of the tube, which are called 
somites. The first pair of somites appears at about the twentieth day 
after fertilization, at the cranial end of the neural tube. More pairs 
appear in the caudal direction, up until about the thirtieth day. 
Mesodermal cells from the somites give rise to most of the skeleton 
and skeletal muscle. 
Blood cell and blood vessel formation actually start at the 
beginning of the third week after fertilization, first in the supportive 
structures of the yolk sac and chorion. Blood vessel formation begins 
in the embryo body about two days later, although blood is not 
formed in the embryo itself until the fifth week. The heart begins as a 
wide blood vessel, which later folds up to develop the chambers of 
the fully formed heart. But even as a tube, the membranes of its cells 
have the electrical and contractile capacity to begin beating in the 
third week, and thus to begin primitive circulatory function with 
blood. During this time the primary chorionic villi elaborate branches 
and form capillary networks and vessels connected with the 
embryonic heart. Oxygen and nutrients diffuse from the maternal 
blood to the embryonic blood through these capillaries, while carbon 
dioxide, urea, and other metabolic wastes diffuse from the embryonic 
blood into the maternal blood. Meanwhile, even firmer connections 
form between embryonic supporting membranes and the 
endometrium, finally completing the development of the placenta. 
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