Figure 1. Timing of transcriptional onset and early embryonic events varies in vertebrates.

Evolutionary distance, embryo morphology, cell division timing and number in pre-gastrulation development for zebrafish (Danio rerio), frog (Xenopus laevis), mouse (Mus musculus), and human (Homo sapiens). X. laevis and X. tropicalis have broadly similar activation timing with respect to cell division number (Yanai et al., 2011). Purple shading density illustrates amount of zygotic transcription. In the fast developing species, fish and frog, cell divisions are rapid until the mid-blastula transition at the 10^th and 12^th cycles. In slow developing species, the initial divisions take place over days. Divisions ‘2’ and ‘2*’ refer to the two asynchronous divisions in the 2^nd cleavage cycle. Xenopus embryo images adapted from (Nieuwkoop and Faber, 1994); Danio rerio embryo images were reproduced from (Kimmel et al., 1995) with permission (John Wiley and Sons).