ZYGOTE PERIOD (0-3/4 h)

Modified from: Kimmel et al., 1995. Developmental Dynamics 203:253-310. Copyright © 1995 Wiley-Liss, Inc. Reprinted only by permission of Wiley-Liss, a subsidiary of John Wiley & Sons, Inc.

The newly fertilized egg is in the zygote period until the first cleavage occurs (Fig. 3), about 40 minutes after fertilization. The zygote is about 0.7 mm in diameter at the time of fertilization. We include only a single stage, however many changes are occurring, and one could easily subdivide the period (see Hisoaka & Battle, 1958).

Stages During the Zygote Period

1-cell stage (0 h): The chorion swells and lifts away from the newly fertilized egg (Fig. 3A). Fertilization also activates cytoplasmic movements, easily evident within about 10 minutes. Non-yolky cytoplasm begins to stream towards the animal pole, segregating the blastodisc from the clearer yolk granule-rich vegetal cytoplasm (Fig. 3B). This segregation continues during early cleavage stages.