Embryonic history of the aortic arches in mammals. 



393 



The figures of His (1880 — 1885) on the human embryo of the first 

 and second months, deserve special mention since they represent actual 

 reconstructions, and, up to the present time, have furnished a basis for 

 the text-book illustrations of stages in the history of the aortic arches in 

 the human embryo. 



Van Bemmelen, in 1886, noted the 

 presence of two pairs of aortic arches behind 

 the fourth pair in embryos of Lacerta, 

 TropidoHotus, and the chick, thus making a 

 total of six pairs for these vertebrates. Of 

 these arches, the fifth attains but slight size 

 in the chick, and soon disappears, and the 

 sixth gives rise to the pulmonary arteries. 

 It is to be regretted that his descriptions 

 were not accompanied by figures. 



In his papers of 1885, 1886, 1 and 2, 

 1887 and 1 893, he furnished an excellent history 

 of the aortic arches in Reptilia showing the 

 presence of a rudimentary fifth arch in that 

 group. 



Boas (1887) offered a most interesting 

 hypothesis, based upon the investigations of 

 Van Bemmelen, and his own earlier obser- 

 vations on Dipnoans and Amphibia. As 

 indicated above, he had found a rudimentary 

 fifth arch in the Dipnoi and the larvae of 



certain Amphibia. This fifth arch is transitory in all the Amphibia ob- 

 served, excepting occasionally the salamander, and as a consequence, the 

 pulmonary artery arises in connection with the sixth aortic arch. Now 

 since the third and fourth arches in Amniota, as in Amphibia, become 

 respectively the carotid and systemic, it follows that, if there are but five 

 pairs of aortic arches in Amniota, the pulmonary artery must arise from 

 a different pair of arches than in Amphibia. This led Boas to the 

 suspicion that observers have overlocked a fifth arch for Amniota, lying 

 between the systemic and pulmonary arches. He prepared diagrams based 

 on this assumption, and predicted that a fifth arch would be also discovered 

 in mammals. 



Mackay (1888), gives diagrams illustrating the transformation of the 

 arterial arches in reptiles and birds differing from those of Rathke and 

 Sabatiee, especially in reference to subclavians and carotids. He main- 

 tains that the subclavian arises from the ventral portion of the third arch, 

 and that both external and internal carotids are derived from the dorsal 

 end of the third arch. Since his results do not come under discussion in 

 this paper, it may be remarked, in passing, that Hochstettee showed, 

 in 1890, that the avian subclavian of Mackay and his predecessors is, in 

 reality, the secondary and not the primary artery supplying blood to the 

 wing-bud. 



Mall (1888) published reconstructions of the branchial region in the 



Fig. Ü. 



Sabatler's diagram , published 



1874, to show the transformation 



of the aortic arches in birds. 



