TRANSFORMATION OF THE AORTIC-ARCH SYSTEM DURING THE 

 DEVELOPMENT OF THE HUMAN EMBRYO. 



INTRODUCTION. 



It has been the experience of embryologists that the more carefully the anatomy 

 of the mammalian embryo is studied the more apparent it becomes that the various 

 structures of the body do not in any complete sense recapitulate their phylogenetic 

 history. The form which the recapitulation assumes is by no means precise, 

 since it is much foreshortened and distorted. Because it is so strikingly suggestive 

 of the organization of a gill-bearing ancestor, the system of aortic arches has 

 constituted a favorite illustration for the recapitulation theory; and although it 

 has become evident, through the work of Tandler and others, that these vessels 

 fall far short of repeating their ancestral history, nevertheless all descriptions of 

 their development have been dominated by this theory, and the reader carries 

 away in his memory schemata taken bodily from the branchial-arch system of the 

 anamniotes. 



A natural accompaniment to a belief in strict recapitulation was the conception 

 of Rathke (1843) as to the nature of arterial developmental changes. He repre- 

 sented the transformations in the aortic-arch system as being the result of the 

 dropping out of certain definitely fixed segments, as though the system were made 

 up of hard and fast units existing of and for themselves. His well-known diagram 

 has perhaps done more harm than good by forcing implications as to the manner of 

 arterial development that are incongruous with what one actually finds in the 

 mammalian embryo. He left out of account the formative influence of one develop- 

 ing organ upon another, which we are gradually coming to recognize as a factor of 

 great importance. It is being repeatedly demonstrated that the vascular system 

 is especially responsive to the conditions of its environment. A more striking 

 illustration of the influence of adjacent structures could scarcely be found than 

 occurs in the aortic-arch system. During the time that the pharynx, with its 

 pouches, is interposed between the heart and the dorsal aorta, the channels of the 

 arterial blood-stream, in form and position, reflect its relief; but as the pharynx- 

 changes its form and the heart descends into the thorax, a new environment is 

 created, which brings about a complete alteration in the branchial pattern and the 

 development of an entirely new arterial arrangement. No precise method of 

 nomenclature for the developing arteries has as yet been evolved. There is lack 

 of precision in using the name given to the adult vessel for the series of short stages 

 of increasing completeness which precede the definitive vessel. The term primitive 

 may be used to call attention to the incompleteness, but frequently, as in the case 

 of the right subclavian, several successive terms would be warranted. 



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