ANALYSIS OF CELL PROLIFERATION DURING EARLY EMBRYOGENESIS

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Abstract
Cell proliferation was examined during early embryogenesis of the newt (Triturus pyrrhogaster) by various methods. After the 2-cell stage, at 23.degree. C, the blastomere (cell) number/whole embryo increased logarithmically until the mid-blastula stage (for about 19 h) and the rate of increase slowed down in and after the late blastula stage. The synchronous cleavage of the blastomeres at the animal pole continued for 18 h until the 12th cleavage (mid-blastula) and the transition from synchronous to asynchronous division occurred abruptly at and after the 13th cell division (late blastula). The presumptive neuro-ectoderm consisted mainly of cells of the 15th generation (G-15) at the onset of gastrulation (pigment stage). The number of ectodermal cells of the early gastrula (stage 12a) nearly doubled during gastrulation at the presumptive neuro-ectoderm. This means that most of the ectodermal cells were in G-16 at the end of gastrulation. Both mitotic activity and the rate of cell increase gradually diminished during gastrulation in the ectoderms of both the presumptive neural and epidermal regions and there were evidently significant differences in both activities between the neuro-ectoderm and the epidermal ectoderm after stage 13b: the epidermal ectoderm showed greater decrease in the rate of both mitotic activity and cell proliferation than the neuro-ectoderm. The determination of whether ectodermal cells will differentiate into neural cells or epidermal cells occurred during G-15 or G-16 in normal primary induction.