Most animals go through a common series of embryological stages,
including a ball-shaped blastula, then a gastrula
stage during which germ layers (ectoderm, mesoderm, endoderm)
are formed. Neurulation (formation of the nervous system)
follows in vertebrate embryos.
Embryonic development is only part of ontogeny,
a life-long process of prenatal development, birth, maturation, adulthood,
and senility.
An organism in its early developmental stages is called an
embryo; embryonic stages are similar in many animal species. A developing
mammal is called a fetus from organ formation until birth.
Development in amphibians:
- Early divisions: The fertilized egg (zygote) contains
stored food (yolk). The first cleavage in most animals runs vertically,
from pole to pole. The second cleavage is usually vertical also, at right
angles to the first. The third cleavage is horizontal. Cells of the upper
half (animal hemisphere) contain less yolk and divide faster than cells of the
yolk-rich lower half (vegetal hemisphere).
- Blastula: a hollow ball of cells with a cavity (blastocoel).
All animals go through a blastula stage.
- Gastrulation: Cells begin to invaginate (tuck in) along the
dorsal lip to form a new cavity, the archenteron ("primitive gut").
The entrance to this cavity is called the blastopore.
- Neurulation: Nervous system forms from a pair of neural folds
which close over. Mesodermal somites (muscle blocks) form soon
afterwards.
- Development of the sense organs, gill slits, and circulatory system.
- Hatching into a gill-breathing larva (tadpole stage).
- Metamorphosis into an adult, controlled by thyroid hormone.
Germ layers (usually formed during gastrulation):
- Ectoderm: Forms the outer layer of the body (epidermis of the skin,
skin glands, hair, etc.); also the nervous system (brain, nerves, etc.)
- Mesoderm: Forms most muscles and bones, plus the entire circulatory,
excretory, and reproductive systems, plus major parts of many other organs.
- Endoderm: Forms the inner lining of the gut, plus the lungs, trachea,
middle ear, liver, and pancreas.
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