50 AORTIC-ARCH SYSTEM IN THE HUMAN EMBRYO. 



In this study the successive changes in the arch system and the arteries that 

 evolve from it have been followed through human embryos ranging in length from 

 1.3 to 24 mm. The gaps in the developmental process are small, since 29 stages 

 are included in the series. Microscopic study was supplemented in each case by 

 models made by the wax-plate method. Several of these reconstructions were 

 already in the laboratory, having been prepared in connection with other studies, 

 notably those of Ingalls, Bartelmez, Davis, Evans, and Streeter. Plaster casts 

 were made from some of the plates by Mr. O. 0. Heard, whose skilful aid is greatly 

 appreciated. The colored figures were the work of Mr. J. F. Didusch and were 

 drawn from models. I am much indebted to him for their excellent rendering and 

 for further assistance in reconstructing some parts. 



I should like also to express my thanks to Dr. C. H. Heuser for his courtesy 

 in permitting the control of the observations on models by a comparison of his 

 beautiful india-ink injections of pig embryos. It is a pleasure to express my obli- 

 gation to Dr. G. L. Streeter for the interest and encouragement he has shown in this 

 work and for his courtesy in placing freely at my disposal the material and the 

 facilities of the Carnegie Embryological Laboratory. 



BRANCHIAL PHASE OF AORTIC ARCHES. 



In following the growth changes of any structure, it is desirable to have some 

 scale of general body development to which its successive stages may be referred. 

 The myotomes serve the purpose for only a short time. Body-length, though 

 available during the entire period-, is unsatisfactory as a criterion, since it shows 

 fluctuations depending upon the degree of development, individual variation, 

 the state of preservation, and the curvature of the body. In table 1 the embryos 

 are arranged in the order of then arterial development, and the age at the end of 

 various developmental phases has been approximated according to Mall's (1912) 

 curve of body-length and age. Because of the large number of embryos upon 

 which the estimates are based, they probably closely approach the correct figures. 



The transformations of the aortic-arch system progress through two strongly 

 contrasting phases. The first we may term the branchial phase, since the vessels 

 at this time approximate a pattern which in lower vertebrates is frequently the 

 precursor of the arteries supplying the gill apparatus. The second or post-branchial 

 phase is characterized by the replacement of the branchial by the adult arterial 

 arrangement. For convenience, the breaking of the right pulmonary arch will be 

 considered as marking the boundary between the two. Though some components 

 of the system undergo involution while the arch is still functioning, it is the inter- 

 ruption of the arch that initiates a general disintegration. 



Beginning with the establishment of the first arch, the branchial phase lasts 

 about 22 days. The post-branchial period, in the strict sense, endures for nearly 

 28 years, if this be taken as the growth interval for man. Yet a human embryo of 

 24 mm. has large arteries in the cranial portion of the body which differ only in 

 minor features from the adult condition, since the vital changes of the second phase 

 are over within two weeks from its beginning. 



