Embryonic history of the aortic arches in mammals. 409 



whether or not this forms a part of the external carotid, since in 

 one of the embrj^os of my series (perhaps not typical) its connection 

 with the second arch was lost. 



The second and third arches arise almost simultaneoush' bnt 

 the second arch reaches its maximum size and degenerates to a 

 considerable extent, before the third has reached its greatest dia- 

 meter. 



Variations occur in the manner of appearance of the second 

 arch, both in individual embryos, and in the two sides of the same 

 specimen. In the majority of cases dorsal and ventral buds of 

 varying lengths were formed as Tandlek has described for other 

 forms. Beside these rudiments, a series of disconnected sinuses 

 appears in the middle region of the vessel to be formed. This 

 portion of the arch is the last to lose the reticulated appearance 

 of its walls, and the first to degenerate. Degeneration begins before 

 the sixth arch is formed, and seems to be accomplished by an in- 

 growth of mesoderm which, in the middle portion of the vessel, 

 divides it into sinuses and narrows its channel. Dorsal and ventral 

 remnants of the second arch persist in all the stages observed. 

 (Compare with pig embryos.) 



The third arch forms in a manner similar to the second, and 

 at about the same time. Individual variations also occur in its 

 manner of formation. After reaching its maximum size, the processes 

 of growth are such that the third arch and the dorsal aorta anterior 

 to it become of about equal calibre. In an embryo of twelve and 

 one-half days a marked decrease in the size of the dorsal aorta 

 between the third and fourth arches is evident. 



The fourth arch forms rapidly in the eleventh day, and at twelve 

 and one-half days the vessel of the left side is slightly larger than 

 that of the right. 



The foregoing observations furnish further evidence that a 

 rudimentary fifth arch exists for the rabbit as Zimmeemann has 

 described. No complete fifth arch was found but I was not in 

 possession of a closely graded series of embryos in the eleventh and 

 twelfth days, in which to follow the history of this vessel. 



The pulmonary arches are the last to form (with the exception 

 of the fifth pair), and each gives rise to a pulmonary artery as 

 Bremer has described. At twelve and one-half days the pulmonary 

 arteries are symmetrical on both sides. This condition precedes the 

 asymmetrical arrangement represented in the diagrams of Rathke. 



