Section through Xenopus blastula section, showing prospective fates following gastrulation. This occurs because, although cleavage is holoblastic, division is slower through the yolky vegetal hemisphere Īnd this pattern continues with subsequent divisions being more frequent in the animal hemisphere, which leads to cells there being smaller than in the vegetal hemisphere.įigure 4. The third division is at right angles to the first two (latitudinal), and approximately equatorial except that it is somewhat towards the animal pole. The first cell division goes through the poles of the zygote (meridional), as does the second but at right angles to the first, to produce four approximately symmetrical cells. Cleavage and BlastulaĬleavage is holoblastic: the first divisions extend right through the zygote. Such that part of the underlying animal hemisphere becomes visible as a ‘grey crescent’.ĭue to this rotation, sperm entry (which always occurs in the animal hemisphere) specifies the dorsal-ventral (back-front) axis of the embryo,īecause gastrulation begins opposite sperm entry, on the same side as the grey crescent. Sperm entry and completion of meiosis prompts the cortical cytoplasm to rotate by about 30° relative to the deep cytoplasm, In the haploid egg meiosis is halted at metaphase II, and on fertilisation meiosis completes to yield the diploid zygote. The inner cytoplasm of the animal hemisphere is darker than that of the vegetal hemisphere Īnd, corresponding with this, the animal cortical cytoplasm is pigmented, whereas the vegetal cortical cytoplasm is not. In Xenopus the cytoplasm is in two distinct parts, inner and outer (or cortical). Egg and fertilisationĪmphibian eggs are generally large, with a substantial quantity of yolk. See here for a brief outline of early vertebrate embryonic development. Most embryological studies have been on frogs because their development can be observed easily,Įspecially the African clawed frog ( Xenopus laevis) because it is easy to stimulate egg laying,Īnd the following description is based mainly on this species. caecilians (without limbs) so resemble worms or snakes.There are three groups of present-day amphibians: See here for a classification of the main groups of vertebrates. See here for a glossary of embryological terms.
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