ABSTRACT
Mitotic counts have been made in the area pellucida endoderm of 10 chick embryos.
In the head process stage embryos there was no evidence that any region of the endoderm consistently possessed a mitotic rate which was statistically significantly different from that of any other.
In all the early somite stage embryos a region possessing a high mitotic rate was found in either the foregut floor or the presumptive foregut floor. It is suggested that as the histological structure of the developing foregut floor of the embryo changes from cubical to columnar, and the individual cells become narrower, a proliferation is needed to maintain the width of the region. There is no evidence that this high mitotic rate is necessary for the morphogenetic movements leading to the ventral closure of the foregut.
There is no evidence to suggest that a lowering of the mitotic rate plays any part in the thinning of the roof of the developing foregut.
