396 Hakbibt Lehmann, 



continuous with the vessel. Where degeneration begins late, as in the 

 right fourth and sixth arches, there is first a diminution in calibre owing 

 to thickening of the walls. After this, the reduced vessel persists for 

 some time as a filiform structure. Tandler found in certain rat embryos 

 a structure which he thinks corresponds to the fifth arch described by 

 Zimmermann, but which does not agree with Zimmermann's descriptions. 

 The structure referred to occurs in the rat, as a short vertical communi- 

 cation between the fourth and sixth arches, toward their dorsal ends. In 

 stage VII of his series, Tandler observed a short spur from the posterior 

 side of the fourth arch, which was directed caudalwards and ended blindly 

 midway between the fourth and sixth arches. This he considers the 

 beginning of the fifth arch. In his stage IX, the posterior wall of the 

 fourth arch bends backward until it comes in contact with the upper wall 

 of the sixth arch, and appears to unite with it for a very short distance. 

 This backward extension of the fourth arch corresponds in position to 

 that of the spur described in stage VII. In stage XI, a distinct short 

 vessel takes its course in a vertical direction from the fourth to the sixth 

 arch. The position of this vessel is similar to that of the spur in stage VII 

 and the union noted in stage IX. In his entire series of rat embryos 

 Tandler found no other vessel corresponding to a fifth arch, but all 

 observations of this vessel have shown it to vary considerably, both as to 

 origin and course, in different species, and in individual embryos. He 

 observed no fifth branchial pouch in the rat, but considers this fact of 

 minor importance, since the branchial pouches of this species differ in 

 many points from those of other forms. This supposed fifth arch in the 

 rat appears in all cases observed by Tandler, after the division of the 

 Conus arteriosus, while all the other arches are formed before this separation. 

 This agrees with the observations of Zimmermann and also those of 

 Tandler for the embryos of other species. He describes in detail the 

 formation of the various branches of the carotids, in both the rat and the 

 human embryo, which discussion need not be taken up in this paper. 



He found a distinct fifth aortic arch in two human embryos. In one 

 of these embryos division was taking place in the conus arteriosus, and 

 in the other the separation into aortic and pulmonary trunks was complete. 

 In each embryo, the fifth arch passed obliquely from the aortic trunk 

 backward and upward, to the sixth arch, uniting with the latter at a 

 distance about one-third of its entire length, from its dorsal end. In either 

 case, this arch does not fully agree in its course with the fifth arch de- 

 scribed by Zimmermann for the human embryo, but takes a course more 

 nearly like that which he has described for the fifth arch of the 

 rabbit. However, instead of emptying into the dorsal aorta near 

 the sixth arch, as described by Zimmermann for the rabbit, in 

 Tandler's human embryos, it empties into the sixth arch itself. In 

 one of these specimens a distinct diverticulum of the fourth branchial 

 pouch was visible between the fifth and sixth arches. A comparison of 

 individual accounts of the fifth arch shows that it is variable in its history. 

 The "island-form" structuies which Tandler found in the region of the 

 union of the fourth arch with the dorsal aorta, and also at the ventral 



