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From this moment the embryo is called a gastrula, and the concave, visible from outside, is the emerging blastopore. Over the upper edge of the blastopore, called the dorsal lip, a migration of cells occurs, which derives from the central area of the gray crescent. The process of involution takes place – the dorsal lip turns in upon itself.
Diagram of stages of embryo development to a larval and adult stage. In developmental biology, animal embryonic development, also known as animal embryogenesis, is the developmental stage of an animal embryo. Embryonic development starts with the fertilization of an egg cell (ovum) by a sperm cell (spermatozoon). [1]
A human infant, the best-known altricial young. In birds and mammals altricial species are those whose newly hatched or born young are relatively immobile, lack hair or down, are not able to obtain food on their own, and must be cared for by adults; closed eyes are common, though not ubiquitous. Altricial young are born helpless and require ...
Dogs go through prenatal development before they are born, just like all animals do. The germinal stage, the embryonic stage, and the fetal stage are the three phases that make up this development. The first stage of prenatal growth in dogs is known as the germinal stage, which starts at fertilization and lasts for about two weeks.
In most animals, the embryo is the sessile initial stage of the individual life cycle, and is followed by the emergence (that is, the hatching) of a motile stage. The zygote or the ovum itself or the sessile organic vessel containing the developing embryo may be called the egg.
Amniotes are distinguished from the other living tetrapod clade — the non-amniote lissamphibians (frogs/toads, salamanders/newts and caecilians) — by the development of three extraembryonic membranes (amnion for embryonic protection, chorion for gas exchange, and allantois for metabolic waste disposal or storage), thicker and keratinized ...
A more developed form of viviparity called placental viviparity is adopted by some species of scorpions [23] and cockroaches, [24] certain genera of sharks, snakes and velvet worms. In these, the developing embryo is nourished by some form of placental structure. The earliest known placenta was found recently in a group of extinct fishes called ...
The cells at the medial hinge point become wedge shaped. In some contexts, such as in Xenopus frog embryos, this cell shape change appears to be due to apical constriction. [28] [29] However, in chickens and mice, bending at this hinge point is mediated by a process called basal wedging, rather than apical constriction.