410 Haeeiet Lehmann, 



The subclavians of the rabbit arise as Hochstetter has de- 

 scribed, from the aortic trunk, and in the process of development, 

 they come to lie gTaduallj' nearer the union of the aortic roots. 



B. The aortic arches in pig einhryos. 



Embryo of the twentieth day (early). The pig- and 

 rabbit series overlap. The youngest pig- embryo in which a recon- 

 struction of the aortic arches was made is shown in external view 

 in Fig-. 13, Plate 22. From a comparison with the figures of Keibel, 

 it is estimated to be rather early in the twentieth day of development. 

 Three well developed branchial pouches are present, and a fourth 

 is clearly outlined. 



Fig. 14, Plate 23, represents a reconstruction of the aortic arches 

 for the riglit side of the same embryo. Two complete arches, the 

 third and fourth, are present, but portions of the first, second and 

 sixth arches also exist. The first two are in the process of de- 

 generation, but the sixth is not yet fully formed. All these vessels 

 are substantially symmetrical on the two sides of the embryo. The 

 remnant of the aortic arch of the mandibular bar has completely 

 lost its connection with the truncus arteriosus, and consists of a 

 slender branch retaining connection with the dorsal aorta, terminating 

 about midway between that vessel and the truncus, and surrounded 

 in its ventral portion by numerous small disconnected sinuses. The 

 sinuses indicated at x and y in Fig. 14, Plate 23 are not connected 

 with the truncus arteriosus, but occupy a position in the visceral 

 arches similar to a ventral remnant of the first arch in the rabbit 

 (cf. Fig. 8 and 10, Plate 23). 



The hyoid arch is more nearly complete. A dorsal rudiment 

 slightly larger than that of the first arch, leaves the dorsal aorta 

 as a single vessel, but after a short distance its channel becomes 

 divided into several smaller ones. These again merge into a single 

 small vessel which terminates a short distance above the large 

 forward extension of the truncus arteriosus. From the anterior wall 

 of the prolongation of the truncus, a slender and short vessel extends 

 upward and laterally into the hyoid arch. The vessels from the 

 two sides join the truncus close to each other and near the median 

 plane. Between this ventral rudiment of the second arch, and the 

 larger dorsal remnant, lie numerous small disconnected pockets in 

 the mesoderm. There is a prominent prolongation of the truncus 

 in front of the third arch. 



