A re-examination of mitotic activity in the early chick embryo.

As a result of extensive mitotic index analysis in colchicine-arrested chick embryos during gastrulation, it was ascertained that the primitive streak is a region of elevated mitotic index as compared to the surrounding tissue. Along the cephalo-caudal axis, the embryo displays two large peaks of mitotic index, one at the posterior end of the primitive streak and the other just anterior to Hensen's node. The length of the various phases of the mitotic period was determined in vitro by time-lapse filming, and the colchicine-arrested mitotic indices in vivo and in vitro were determined and compared for various regions. Some observations regarding the orientation of mitotic spindles and abnormal mitosis in vitro are also included, and the relevance of the above observations to early embryonic development is discussed.