Category : 12th Class
Post gastrulation involves two main process. Neurulation is process of laying the neural plate to form the nervous system. The establishment of the germ layers initiates the final phase of embryonic development, i.e., organogenesis. The latter involves differentiation and specialization of groups of cells in the individual germ layers. The cells of such groups change their form and give rise to morphologically recognizable tissues and organs of the new individual. The groups of differentiated cells separate from their germ layers in an orderly manner and with unique precision. Separation of the differentiated cell groups may occur by folding off from the germ layer or by migration of cells individually and reaggregation at a new place. In this manner, the primordial cells of the germ layers gradually and accurately give rise to the tissues and organs of the offspring.
By four weeks after fertilization, the embryo has a simple heart, limb buds and eye rudiments. It also has a tail and pharyngeal pouches, the vestiges of its early vertebrate ancestors that disappear later in development. After the second month, the embryo is recognizable as a primate. From this stage on, the embryo is often called foetus.
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