Modified from: Kimmel et al., 1955. Developmental Dynamics 203:253-310. Copyright © 1995 Wiley-Liss, Inc. Reprinted only by permission of Wiley-Liss, a subsidiary of John Wiley & Sons, Inc.

Table 1: Periods of Early Development

Period - h - Description

Zygote - 0 - The newly fertilized egg through the completion of the first zygotic cell cycle

Cleavage - 3/4 - Cell cycles 2 through 7 occur rapidly and synchronously

Blastula - 2 1/4 - Rapid, metasynchronous cell cycles (8, 9) give way to lengthened, asynchronous ones at the midblastula transition; epiboly then begins

Gastrula - 5 1/4 - Morphogenetic movements of involution, convergence and extension form the epiblast, hypoblast and embryonic axis; through the end of epiboly

Segmentation - 10 - Somites, pharyngeal arch primordia, and neuromeres develop; primary organogenesis; earliest movements; the tail appears

Pharyngula - 24 - Phylotypic-stage embryo; body axis straightens from its the early curvature about the yolk sac; circulation, pigmentation, and fins begin development

Hatching - 48 - Completion of rapid morphogenesis of primary organ systems; cartilage development in head and pectoral fin; hatching occurs asynchronously.

Early larva - 72 - Swim bladder inflates; food-seeking and active avoidance behaviors