Because of its importance in early embryogenesis, the developmental history of the yolk syncytial layer (YSL) of Fundulus has been investigated in detail. As in other teleosts, the Fundulus YSL forms mainly by collapse of certain marginal blastomeres which then merge with the cytoplasm of the yolk cell peripheral to the blastoderm. Nuclei enter the yolk cell from these open blastomeres variably during cleavages 8-11, but most frequently at cleavages 9 and 10. After entry, the first nuclei divide five times and later nuclei divide with them. Thus, nuclei that have invaded at the next cleavage divide four times, etc. When the first YSL nuclei cease dividing, all other YSL nuclei cease dividing with them. These YSL mitoses occur in metachrony. Two or more metachronous waves progress through the YSL cytoplasm at each mitosis. After each nuclear division, the YSL increases in width and its nuclei are quite evenly spaced. After the 5th and last mitosis, when the YSL is at its widest, it contracts in its animal-vegetal axis. This slow contraction has two major effects: 1) narrowing of the YSL, accompanied by crowding of its nuclei and their disappearance beneath the blastoderm to nucleate the internal YSL; 2) epibolic expansion of the I-YSL and the blastoderm, followed soon after by other cell movements of gastrulation. This YSL transition, therefore, sets the stage for the onset of gastrulation. It is preceded by increased duration and variability of succeeding mitoses and, in particular, duration of their interphases, a decrease and deceleration in the rate of the last metachronous waves, and, finally, by the complete cessation of mitosis and the entry of YSL nuclei into permanent interphase.