The first signs of cell motility appear in Fundulus toward the end of cleavage, after cleavages 11 and 12. When blastomeres cease cleaving, their surfaces undulate and form blebs. At first, these blebbing cells remain in place. Gradually thereafter they begin movement, with blebs and fllolamellipodia serving as organs of locomotion. Non-motile cleaving blastomeres have thus differentiated into motile blastula cells. This transformation corresponds to the midblastula transition of amphibian embryos.
Gastrulation in Fundulus begins with vegetalward contraction of the external yolk syncytial layer. This causes narrowing of the E- YSL and initiates the epibolic expansion of the blastoderm. Convergent movements of deep cells within the blastoderm begin toward the end of this contraction. The YSL forms as a result of invasion of the yolk cell cytoplasm by nuclei from open marginal blastomeres during cleavage. These YSL nuclei then undergo five metachronous divisions. After this, they divide no more. YSL contraction begins approximately 1.5 hours after cessation of these divisions (21–22°C). This cessation of nuclear divisions is preceded by a gradual decrease in rate. (1) The duration of each succeeding mitosis increases steadily and often some nuclei do not divide at mitosis V. (2) The duration of interphases between succeeding mitoses also increases, but to a much greater degree, and the longest interphase by far is the last one, I-FV, between mitoses FV and V. (3) The mitotic waves responsible for mitosis V move much more slowly than those for the first four mitoses and invariably decelerate. This gradual cessation of YSL nuclear divisions clearly sets the stage for the contraction of YSL cytoplasm and thus the beginning of gastrulation. We call this the YSL transition. It is not to be confused with the midblastula transition, which occurs 3–4 hours earlier. The MBT commences cytodifferentiation; the YSL transition commences morphogenesis.