Development of General Body Form
Growth in the midline or axial region of the trilaminar embryo is greater than that occurring in peripheral areas and as a consequence, the peripheral areas are gradually overgrown and folded under the central area to form the body folds. The division of the conceptus into embryonic and extraembryonic areas, which first became apparent in the bilaminar embryo, is greatly accentuated by the appearance of body folds. The head fold is the first to appear and is produced as a result of craniocaudal elongation caused by the rapidly developing notochord and central nervous system; the tail fold appears a short time later as a result of the same craniocaudal growth process. Comparable but less distinct lateral body folds gradually appear on each side.
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