A study of the histogenesis of elastic tissue in the embryonic ductus arteriosus of Sus scrofa is in accord with the theory that elastic fibrils are directly differentiated in the outlying portion of the protoplasm of the early connective tissue cell.

In the occlusion of the postfetal ductus arteriosus of Sus scrofa there is early a hypertrophy of the internal elastic membrane. Subsequently there takes place a marked delamination of the thickened internal elastic membrane in the production of new and independent elastic fibers and lamellæ. The formation of new elastic fibers from preformed elastic tissue is most abundant where the postfetal contraction of the ductus arteriosus is least marked. These new elastic fibers play an important part in the occlusion of the lumen of the postfetal ductus.

Aside from the extensive formation of elastic fibers from preformed elastic tissue, in the occlusion of the lumen of the postfetal ductus arteriosus of Sus scrofa, there are also some elastic fibrils formed from non-elastic elements, apparently from connective tissue cells.

In some recent preliminary work on ligations of the common carotid artery there was found, after an interval of from eight to twelve days, at some points between the ligatures, a slight but obvious cellular thickening of the so-called subendothelial stratum. Some of these connective tissue cells may have wandered from the other coats of the vessel, through the inner elastic membrane into the subendothelial stratum; others may have proliferated from cells in situ. Specific stains revealed near the periphery of some of these cells, i. e., in the outlying portion of the exoplasm, very delicate elastic fibrils, apparently the product of protoplasmic activity.

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