Carnegie Stage 11: Heart and Chest
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In the second section, the foregut is indicated. It is the continuation of the oropharynx, and part of the developing gut. The urogenital ridge is the area of the embryo which will make up most of the urinary and reproductive systems. Finally, the heart bulge is the area in which the heart grows and develops from a two-chambered heart to a three- and finally four-chambered heart. The heart bulge in this embryo is only connected by a narrow stalk to the main portion of the embryo. In humans, the heart begins to pump on day 22 of development. It is likely that the heart of this embryo was already beating too.
In fish, the heart remains two-chambered (consisting of one atria and one ventricle), whereas in amphibians the heart is three-chambered (two atria and one ventricle). Mammals such as dolphins and humans have a four-chambered heart consisting of two atria and two ventricles.
