More specifically, they form from endocardial tubes, starting at day 21.
2.
Haeckel argues that certain features in embryonic development are conserved and endocardial tube are considered caenogenetic features.
3.
Two endocardial tubes form here that fuse to form a primitive heart tube known as the tubular heart.
4.
In this area, two separate angiogenic cell clusters form on either side and coalesce to form the endocardial tubes.
5.
By the third week of embryological life, the endocardial tubes have developed a swelling in the part closest to the heart.
6.
As embryonic folding continues, the two endocardial tubes are pushed into the thoracic cavity, where they begin to fuse together, and this is completed at about 22 days.
7.
As folding of the embryo in the horizontal plane initiates in the 4th week of gestation, the endocardial tubes meet in the midline to form the primitive heart tube, which will eventually develop into the histologically definitive endocardium.
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