The splanchnic mesoderm forms the heart tube. The heart tube is a non proliferating structure. Complete formation of the heart however, requires addition of cells. Proliferation rate is exceptionally high in the mediocaudal region of the heart field. It was hypothesized that proliferation in heart field progenitors plays a role in heart development and inhibition of the process will cause congenital heart defects. To test this hypothesis, microinjection of an anti mitotic drug and DiI in the heart field, on the right side of the embryo was carried out. Immunohistochemistry and Fluorescent imaging were used to detect abnormalities in the heart. Data shows that on inhibiting proliferation, the right atrium of the developed heart is either completely missing or is extremely reduced in size and the length of the outflow tract is dramatically decreased.