Refinement of Harrison's normal table for the morula and blastula of the axolotl

Summary Films of cleaving embryos of the axolotl,Ambystoma mexicanum, taken by the double camera technique, were used in order to arrive at a more detailed staging based on quantitative criteria. Drawings were made of the animal hemispheres just prior to the start of each cleavage cycle from the 6th to the 15th cleavage. The number of cells intersected either by the (apparent) egg diameter (up to the 10th cleavage) or by half the diameter (from the 10th cleavage onwards) was determined. The cell numbers for each cleavage cycle did not overlap with those of the previous or succeeding cycles. On the basis of these cell numbers, in place of the four Harrison stages 6–9, 10 successive stages were established each of which corresponds to one cleavage cycle.