The transparent embryos develop outside the mother’s body.
These small fish (2–4 cm long) are easy to breed in the laboratory in large numbers.A second vertebrate model, the zebra fish Danio rerio, has some unique advantages.Their embryos develop in the mother’s uterus, hidden from view.Mice are complex animals with a genome as large as ours.Researchers are adept at manipulating mouse genes to make transgenic mice and mice in which particular genes are “knocked out” by mutation.The mouse genome is about 2,600 Mb long with about 25,000 genes, about the same as the human genome.The mouse Mus musculus has a long history as a mammalian model of development.By following all cell divisions with a microscope, biologists have constructed the organism’s complete cell lineage, showing the ancestry of every cell in the adult body.These arise from the zygote in virtually the same way for every individual.Self-fertilization of heterozygotes produces some homozygous recessive offspring with mutant phenotypes.Because individuals are hermaphrodites, it is easy to detect recessive mutations.It is 97 Mb long and contains an estimated 19,000 genes. Only a millimeter long, it has a simple, transparent body with only a few cell types and grows from zygote to mature adult in only three and a half days.The nematode Caenorhabditis elegans normally lives in the soil but is easily grown in petri dishes.It has 180 × 106 base pairs (180 Mb) and contains about 13,700 genes.Sequencing of the Drosophila genome was completed in 2000.However, because first rounds of mitosis occur without cytokinesis, parts of its development are superficially quite different from that of other organisms.There are vast amounts of information on its genes and other aspects of its biology.Embryos develop outside the mother’s body.It has a generation time of only two weeks and produces many offspring.