400 Haeriet Lehmann, 



mandibular pouches. A reconstruction of the left arch, together 

 with the anterior dorsal aorta of the same side, is shown in Fig. 2, 

 Plate 23. These first arches are large vessels of irregular outline, 

 which after leaving the heart diverge and pass upward through 

 the mandibular bar of either side. They lie, for the most part, just 

 ventral to the alimentary canal. The roots of the dorsal aorta in 

 this embryo, as far as they could be traced, viz : to a point opposite 

 the lower portion of the heart, remain separate, and run just dorsal 

 to the fore-gut. The vessels are practically symmetrical for the 

 two sides. The arch on either side curves into the dorsal aorta 

 just above the anterior end of the fore-gut, forming by this union 

 a vessel resembling an inverted U over the anterior portion of the 

 alimentary canal. A small spur is continued forward from the 

 ventral end of this bend. The first aortic arch in this specimen, 

 especially in the region of the heart, is of considerably larger size 

 than the dorsal aorta. In its upper portion numerous constrictions 

 appear in its walls, and slender trabeculae cross its lumen at ir- 

 regular intervals. In the region of its union with the dorsal aorta, 

 an evagination of the fore-gut is nearly surrounded by these vessels. 

 Just in front of the plane passing through the auditory pit, a small 

 lateral evagination arises from the dorsal aorta (Fig. 2, Plate 23). 

 A similar evagination (IP) is present from the posterior border of 

 the first arch. These spurs are difficult to represent in reconstruction 

 since they take an almost lateral direction. They are supposed to 

 possibly represent the beginnings of the second arch. 



To summarize the conditions in this embryo: one complete arch 

 is present, and on account of its reticulated appearance '), may be 

 considered as being still in an unfinished condition. 



In the following stage the mandibular arch is a vessel with 

 much more definitely limited and regular walls, without the trabeculae 

 or constrictions described above. 



It is interesting to note, however, that the first arch is relatively 

 much larger in Fig. 2, than in the following stage (Fig. 4). The 

 two evaginations mentioned are probably buds of the second arch-), 

 since the one from the dorsal aorta corresponds in position with 

 reference to the auditory pit, to that of the dorsal portion of the 

 second arch when completely formed. 



1) cf. VlALLETON for chick, and Maeshall & Bles for frog. 



2) Tandlek on origin of arches. 



