Stage 2

Key events: syncytial divisions 3-8, cytoplasmic clearing

Stage 2 lasts for about 40 min (0:25-1:05 h). During this stage cleavage divisions 3 - 8 take place. A characteristic of this stage is a significant retraction of the embryo from the vitelline envelope (not shown in the movie). This leads to the appearance of two empty spaces (perivitelline space), one at the anterior and one at the posterior pole of the egg. The posterior space will be occupied by the pole cells in stage 3 whereas the anterior space vanishes during stage 5. Retraction of egg cytoplasm begins when four zygotic nuclei are present; they are consistently located in the anterior one-third of the egg, surrounded by cytoplasmic islands (energids) and therefore clearly separated from the yolk particles.

During divisions 3 to 5, the nuclei and their surrounding cytoplasm tend to move posteriorly; by the fifth cleavage, zygotic nuclei occupy an ellipsoid field in the center of the egg, between 20 and 80% egg length (0% egg length = posterior pole).

From this point on the nuclei begin to move peripherally, advancing stepwise, at a rate of about 10 µm per divisional cycle. By the end of the eighth division, the majority of nuclei are evenly arranged on an ellipsoidal surface about 35 µm beneath the membrane. Some nuclei remain centrally located others drop out from the periphery and give rise to the vitellophages (yolk nuclei). Counts performed by Zalokar and Erk (1976) indicate that there is a doubling of the number of nuclei after each division; at the end of the eighth mitosis they found about 200 nuclei populating the periphery and about 50 presumptive yolk nuclei located in the center of the embryo.


Media list
Stage 2, basic fuschsin staining
Stage 2 (0.3 MB)
Stage 2, in vivo
(5.3 MB)