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Embryogenesis is the process by which the embryo is formed and develops. It starts with the fertilization of the ovum, which is then called a zygote. The zygote undergoes rapid mitotic divisions with no significant growth (a process known as cleavage) and cellular differentiation, leading to development of an embryo. It occurs in both animal and plant development, this article will adress the common features among different animals.


The zygote

The egg cell (and hence the fertilized egg) is always asymmetric, having an "animal pole" (future ectoderm and mesoderm) and a "vegetal pole" (future endoderm), it is also covered with different protective envelopes. The first envelope, the one which is in contact with the membrane of the egg, is made of glycoproteins and is called vitelline membrane (zona pellucida in mammals). Different taxa show different cellular and acellular envelopes.


The zygote undergoes rapid cell cycles with no significant growth, producing a cluster of cells the same size as the original zygote. Depending mostly on the amount of yolk in the egg, the cleavage can be holoblastic (total) or meroblastic (partial). The different cells derived from cleavage (up to the blastula stage) are called blastomeres.

In holoblastic eggs the first cleavage always ocurrs along the vegetal-animal axis of the egg, the second cleavage is perpendicular to the first. From here the spatial arrangement of blastomeres can follow various patterns, due to different planes of cleavage, in various organisms:

Cleavage patterns followed by holoblastic and meroblastic eggs
Holoblastic Meroblastic

Blastula and Gastrula

Blastulation begins after the cleavage has produced 128 cells, in this stage the embryo is called a blastula. The blastula is usually a spherical layer of cells (the blastoderm) surrounding a fluid-filled or yolk-filled cavity (the blastocoel).

In mammals blastulation leads to the formation of the blastocyst, which must not be confused with the blastula, even though being similar in structure their cells have different fates.

During gastrulation cells migrate to the interior of the blastula, consecuently forming two (in diploblastic animals) or three (triploblastic) germ layers, the embryo during this process is called gastrula.

  • Among the different animals, different combinations of the following processes occur to place the cells in the interior of the embryo:
    • Epiboly (spreading of a sheet of cells under the external sheet of cells).
    • Ingression (cells move with pseudopods)
    • Invagination
    • Delamination (the external cells divide, leaving the daughters cells in the cavity)
    • Polar proliferation


At some point after the different germ layers are defined, organogenesis begins. The first stage in vertebrates is called neurulation, where the neural plate folds forming the neural tube. Other common organs or structures which arise at the time include the heart and somites, but from now on embryogenesis follows no common pattern among the different taxa of the animal kingdom.

In most animals organogenesis along with morphogenesis will result in a larva. The hatching of the larva, which must then undergo metamorphosis, marks the end of embryonic development.

See also

Last updated: 08-13-2005 06:06:34
Last updated: 10-29-2005 02:13:46