428 Harriet Lehmann, 



structures of this region, that it is difficult to explain just how these 

 conditions are brought about. 



In dissections of stages in which the transformations are 

 practically completed, there is a single brachiocephalic trunk giving 

 rise to the two common carotids and right subclavian, and the left 

 subclavian arises individually farther toward the left, from the arch 

 of the^ aorta. 



I have followed the subclavian s from their first appearance in 

 a rabbit of eleven days, to the oldest stage considered in the series 

 of pig embryos. They arise in the rabbit, at a considerable distance 

 (slightly more than 1 mm) below the union of the aortic roots, but 

 as the embryos advanced in age, these vessels were found to lie 

 nearer the aortic roots. The condition was similar in the series of 

 pig embryos, and the history could be carried further. As the embryo 

 advanced in age, during the twentieth day, the distance between 

 the roots of the subclavians and the union of the aortic roots 

 diminishes until, in an embryo of the twenty-first day, the vessels 

 supplying the fore-limbs arise directly at the union of the aortic 

 roots. In the succeeding stages the subclavians are found arising 

 from both aortic roots, the distance above the union of the latter 

 vessels increasing as the embryos advance in age. This agrees 

 with Hochstetter's description of their development. The shifting 

 in position of the subclavians with reference to the aortic roots, has 

 no doubt given rise to the many apparently conflicting statements 

 with regard to their origin. 



Conclusions. 



1. Six aortic arches arise in both pig and rabbit embryos. In 

 the pig there is a complete fifth arch on about the twenty-first day 

 of development. In this form there are also two entodermal pouches 

 between the fourth and sixth arches. The rudimentary fifth in the 

 rabbit arising at about eleven and one-half days, is less complete. 



2. The aortic arches are formed by spurs from the dorsal aorta 

 and the truncus arteriosus, growing toward each other; sometimes 

 with an intermediate sinus or vessel joining the two. 



3. Eemnants of the first and second transitory arches are 

 persistent in the pig, so that parts of six aortic arches exist simul- 

 taneously. 



4. The transitory vessels exhibit two modes of degeneration; 

 one in which the calibre is reduced by thickening of the walls of 



