Consistently six to seven somites were formed in explants of presomitic mesoderm of 8.5-day to 11.5-day mouse embryos and this number correlated well with the number of somitomeres previously identified in the same tissue by stereo SEM (Tam, Meir & Jacobson, 1982). During this period of development, the size of the presomitic mesoderm varied up to two-fold but the number of prospective somites remained unchanged. This pattern in the presomitic mesoderm was stable with respect to the number and the position of somites that were formed, the craniocaudal sequence and the rate of segmentation. The prospective somite also went through an orderly sequence of expansion and cellular rearrangement prior to segmentation. During the differentiation of the presomitic mesoderm, somites were formed at the expense of somitomeres, suggesting that the somitomere was a morphological manifestation of the prospective somite.