Embryonic Development

EMBRYONIC DEVELOPMENT (germinal development, embryogenesis, embryogeny) encompasses the initial stages of the development of a new individual from the egg, occurring under the protection of egg or fetal membranes, up to its hatching from the membranes or birth. The duration of the embryonic period varies among different animals and depends mainly on the degree to which the developing embryo is provided with nutritive material (as well as on the rate of development). In animals that lay eggs with only an insignificant supply of yolk, embryonic development usually lasts a short time; the young individual hatches in a very imperfect state and often undergoes a complex chain of further transformations thereafter. If it obtains food independently during this time, one speaks of a larval period of development. In animals possessing large eggs with a large amount of yolk, as well as in those viviparous animals in which the nutrition of the developing embryos is provided by the maternal organism, embryonic development usually lasts longer, and as a rule, a much more perfect young animal hatches or is born. Thus, in most fishes and amphibians, embryonic development usually lasts only a few days (large eggs of salmonid fishes are spawned in the winter and correspondingly develop significantly longer), and the hatching individual (fry, larva) is very imperfect and undergoes still significant postembryonic development. On the other hand, in reptiles and birds, which lay large eggs, embryonic development lasts much longer. However, in birds, embryonic development is relatively more rapid (usually 2-3 weeks) due to the higher temperature at which development takes place (incubation of eggs). The hatching young in reptiles and precocial birds is very imperfect and often feeds and searches for food quite independently (reptiles). In mammals, various degrees of complexity of embryonic development are observed. Lower, cloacally respiring mammals lay eggs (12-15 mm in diameter) surrounded by a leathery membrane, which are then carried in a pouch (echidna) or incubated (platypus). Germinal development lasts a short time. Marsupial mammals are viviparous, but a true placenta does not develop in them, and the nutrition of the embryo is not as well provided for as in higher mammals. In marsupials, the embryonic period of development is very short, the young is born extremely imperfect and is then carried in a pouch. Immediately after birth, the young attaches itself to the nipple and then hangs on it for a long time without letting go (the nipple swells and fills the entire oral cavity). In placental mammals, a much closer connection of the embryo with the mother develops—the wall of the allantois closely connects with the serous membrane and vascularizes it; on the "chorion" thus obtained, numerous villi develop, growing into the mucous membrane of the uterus and forming together a special organ of nutrition and respiration of the embryo—the placenta (see), which has a very different structure in various mammals. With such more perfected forms of nutrition, a much longer embryonic development also becomes possible. In this case, the born young often has relatively very large dimensions and is sometimes already very well developed (higher ungulates), although in mammals it is still fed with milk for some time. See Embryo, Mechanics of development, Ontogeny.