Abstract
The changing external features of Xenopus laevis embryos were examined from the first cleavage through the late gastrula as a means to develop quantitative criteria for the determination of stages for this period. During blastulation, the cell number along arcs centered on the animal or vegetal pole and representing one-sixth of a meridian - called the MCNo. - can be used as such a criterion. During gastrulation, the width of the blastopore divided by the diameter of the whole embryo can be used as such a criterion.

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