Embryonic history of the aortic arches in mammals. 



395 



embryo at the close of the eleventh day, and occurred as a posteriorly 

 directed, and very short branch from the middle region of the fourth arch. 

 Again, in 1891, he reviewed the development of the circulation in 

 vertebrates, and gave diagrams showing the transformation of the aortic 

 arches as based upon the presence of six pairs of embryonic arches in 

 vertebrates above the fishes. 



L. car 



R. car 



L. duct. Art 



Duct. Art 

 R. Subcl 



Pulm 



Fig-. H. 



Illustrating the condition of the arterial arches in a foetal marsupial, 8,5 mm long, 

 with especial reference to the pulmonary arteries. After Broom, 1898. 



Maeshall & Bles (1890) published a paper showing in detail, the 

 origin and fate of the difiFerent aortic arches in the frog. This account 

 is valuable as a basis for comparison , in a study of these structures in 

 other vertebrates. 



Beoom (1898) found in a foetal Marsupial {Trichosurus) of 8,5 mm, 

 an interesting stage in the development of the pulmonary arteries in a 

 mammal, shown in Text-Fig. H. This diagram shows the pulmonary 

 arteries arising symmetrically from the two pulmonary arches. From the 

 conditions in tl)is embryo, BßOOM concludes that either Rathke's de- 

 scription of the origin of the pulmonary arteries in mammals is inaccurate 

 or that the Marsupials exhibit reptilian affinities in this particular. 



Tandler (1902) made an important contribution to the history of 

 the aortic arches in mammals, based upon studies of rat and human 

 embryos. The following is a summary of a part of his conclusions for 

 the rat. Concerning the formation of particular arches, he observed that 

 ventral and dorsal spurs develop first, which grow toward each other, 

 and finally unite. The ventral spur usually appears earlier than the dorsal. 

 The process of degeneration takes place in two different ways, depending 

 upon whether the disappearance occurs early or late in the history of the 

 arches. In the first and second arches where degeneration begins very 

 early, the previously complete vessel is broken up, in such parts as 

 eventually disappear, into lacunae, which can no longer be traced as 



