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Fig. 1. Characteristics of Xenopus laevis early development.
(A) The different cell cycles and the external appearance of (a) the
fertilized egg, and (b) two-cell, (c) mid-blastula and (d) early gastrula
stages. Aa and Ab are views from the animal pole, Ac from the side and Ad from
the vegetal pole. (Aa) Cycle 1 is approximately 90 minutes in length and has
G1 and G2 phases. The next 11 divisions have no gap phases and occur every
20-30 minutes. (Ac) At the mid-blastula stage, the embryo consists of 4000
cells, gap phases reappear, the cycle lengthens to 50 minutes and zygotic gene
expression commences. This marks the mid blastula transition (MBT). (Ad) The
following cell cycle is longer (90 minutes), and the 15th cycle marks entry
into gastrulation, which is a period of mitotic quiescence. (B) The
features of the (a) fertilized egg, (b) the mid-blastula and (c) early
gastrula, which are all viewed in section from the side. (Ba) Sperm entry
activates the microtubule polymerization, which drives the rotational movement
of the outer shell of cytoplasm (cortical cytoplasm) away from the sperm entry
point (black arrow). Maximal rotation away from the sperm entry point occurs
at a point at the circumference that subtends a 30° angle from the vegetal
pole (red arrow). A dye mark placed on this spot (where 
14 cell cycles
later the dorsal lip of the blastopore forms) marks the beginning of
gastrulation (red arrow in Bc). Immediately above the dorsal lip is the region
of the Spemann Organizer (shown as a blue trapezoid in Ad). (Bb) At the
mid-blastula stage, the embryo is described as having three regions, the
animal cap, equatorial or marginal zone and vegetal mass, explants of which
are dissected along the broken black lines shown.
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