62 AORTIC- AECH SYSTEM IN THE HUMAN EMBRYO. 



by Tandler (1902) and Lehman (1905). In the figures of Lehman they are repre- 

 sented as being independent of the circulatory system; she regarded them as 

 fragments left behind by the involution of the first and second arches. Dr. Streeter 

 suggests that they may be proliferations of endothelium for the supply of the 

 developing ventral arteries, and thus progressive rather than regressive in nature. 

 No evidence for the degeneration of the endothelium of the two arches was found. 

 It seems probable, therefore, that it is worked over into the capillary net and the 

 larger vessels that succeed them. The regression of small vessels will be con- 

 sidered again with reference to the interruption of the segmental arteries during the 

 formation of the vertebral. 



In the post-branchial period the differentiation of the subpharyngeal region 

 has permitted the ventral artery to develop branches somewhat resembling those 

 of the definitive external carotid. There are, for example, lingual twigs passing 

 between strata of the developing lingual muscles. The artery is now sufficiently 

 withdrawn from the thyroid plexus to have a definite thyroid branch. Other 

 ramifications are already present, and there also may be finer branches given off 

 from the third arch close to it. While the vessel is thus taking form, it is gradually 

 withdrawn from the midline. At the end of the branchial developmental phase 

 it is given off from the third arch near its junction with the aortic sac (fig. 12). 



The process of involution of the two cranial aortic arches and of the develop- 

 ment of the arteries that succeed them has been variously interpreted. The 

 earlier observers did not find the mandibular and hyoid arteries. As material 

 improved and experience increased, these vessels were usually seen only in part 

 and were interpreted as fragments, due to the breaking down of the corresponding 

 arches, rather than as vessels that had taken their place. The point of first inter- 

 ruption has been placed at either end or at some intermediate point, depending 

 probably on the chance conditions of distention of parts of the arteries rather than 

 upon individual or specific differences among mammals. A further study of these 

 changes of vascularization by the injection method is highly desirable. 



POST-BRANCHIAL PHASE. 



( Including embryos up to 25 mm. in length.) 



The disappearance of the aortic-arch sj^stem is amply explained by the separa- 

 tion of the outflow from the heart into two streams and by the changes in the en- 

 vironment of these due to the shifting of the organs among which they must find 

 their way. Though it is necessary, for convenience, to describe the arterial evolution 

 by stages, and to a certain extent independently of the movements, it must not be 

 forgotten that it is a gradual process and is paralleled step by step by changes in the 

 surroundings. 



During the disintegration of the branchial arterial pattern, some of the arches 

 and their connections may be identified for a time; but since their distinguishing 

 characters are largely topographical and their walls differ but little in structure, their 

 individuality is gradually lost and their material worked over and increased to form 



