Mitotic Activity in Chick Embryos at the Primitive Streak Stage

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Abstract
In the present work it has been investigated whether previously recorded regional differences in the average RNA‐concentration of the primitive streak embryo are also connected with differences in mitotic activity. Mitotic indices calculated for the embryo area reveal no decided regional dissimilarities. However, owing to the uncertainty inherent in the interpretation of the index, it was decided to extend the mitotic counts to embryos in which a successive accumulation of mitoses had been produced by means of heat shock and the mitotic poisons, colchicine and oestradiol. From the obvious differences in mitotic activity which appear in such embryos it is clear that cell multiplication is greatest in a region of the embryo comprising the node and adjacent areas on both sides thereof. As differences in cell multiplication are moderate, however, it is presumed that the existing variation in RNA‐concentration must be partly ascribed to differences in protein synthesis within the embryo, associated with the beginning differentiation of the embryo cells. Cytologic and morphologic changes caused by the heat shocks and the mitotic poisons are described.