416 



embryo, the space between the fourth and sixth arches is considerably 

 wider than between the third and fourth. 



This embryo is of special interest as showing variations in the 

 order of development. The rudiments of the first and second arches 

 are more nearly complete than in the embryo shown in Fig, 7, of 

 apparently about the same age — while the pulmonary arches, which 

 were not fully formed in the preceding specimen, are complete in this 

 embryo. In both cases, the dorsal remnants of the first two arches 

 appear to be more persistent than the ventral ones. In Fig. 8, we 

 have five arches nearly complete. It is to be noted in this connection, 

 that His and Tandler both show in the human embryo five complete 

 aortic arches existing at the same time. 



Embryo of the twentieth day (number 3, somewhat 

 older). Fig. 9 shows a reconstruction of the aortic arches of ihe 

 right side of this embryo. In this stage three complete pairs of aortic 

 arches are present, the third, fourth and sixth, and remnants of the 

 first two pairs. The vessels lie in the corresponding visceral pouches. 

 The vessels in the mandibular bar consist of a very slender 

 branch from the dorsal aorta which after leaving the latter vessel 

 bends toward the exterior and then takes a course ventralward through 

 the middle of the visceral arch. Opposite the dorsal end of the cleft 

 which separates the maxillary process of the first arcb from the 

 remaining portion, this small vessel appears to break up into sinuses, 

 which though disconnected, lie close together and car be readily 

 followed. These extend ventralward for a short distance, and then 

 take a general direction toward the anterior prolongation of the 

 truncus arteriosus. The remnants of the first arch u5on either side 

 are essentially symmetrical. 



A vessel of somewhat larger diameter is presert in the second 

 visceral arch of both sides. This artery also bends toA'ard the exterior 

 upon leaving the dorsal aorta, and passes downward ihrough the hyoid 

 arch. A short distance from its origin it divides into two branches 

 which diverge, and finally terminate at about the bvel of the ventral 

 end of the complete portion of the first arch. 



The third arch is somewhat smaller than thi fourth, a reversal 

 of the conditions in the two preceding embryos The fourth arches 

 are large vessels of about equal size on both sides of the embryo. 

 Near its dorsal end, a very slender vessel projects and, independently, 

 enters the aortic root. The sixth arch is at i considerable distance 

 from the fourth. It is similar in form and sze to that in the pre- 

 ceding embryo. It shows two unequal connections with the aortic root. 



