Complete dysraphism in 14 somite human embryo. A contribution to normal and abnormal morphogenesis

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Abstract
The youngest completely dysraphic human embryo of 2.75 mm G.L. has been reconstructed from the microscopic sections and studied in detail. Although the neural folds failed to close throughout the dorsal extent of this 14 somite embryo, the beginning of independent closure took place rostrally at the torus opticus. The last feature has been shown to be the usual event in normal embryos of this stage. Careful examination of other structures than the nervous system reveals only minor abnormalities. Thus, the dispersion of cells of somites I and II as well as loosening of other sclerotomes in this region has begun prematurely. On the other hand, some retardation in the development of the pharynx, heart and nephros has been noted. Specifically, the chorda, the mesenchyme, blood vessels and gut have been found to be normal.The literature pertaining to the closure of the neural folds has been discussed. Also, the reports on experimental inhibition of the formation of the neural tube are discussed with special emphasis on the work of Davis ('42). Davis, J. O. 1942 Photochemical spectral analysis of neural tube formation. University of Missouri, Doctor' thesis. The available evidence refutes the contention that the closure of neural folds is induced by adjacent structures of whatever derivation. It appears plausible as Davis demonstrated in his experiments that certain biochemical reactions within the neural epithelium may be involved in the process of normal closure of the neural folds.Morphological findings in the present embryo do not support most of the previously advanced theories relating to the pathogenesis of dysraphic states.