422 Harriet Lehmann, 



course these arteries approach each other, and become united for a 

 short space. 



The subclavians arise from the aortic roots about ^/o mm in 

 front of the union of the latter, to form the aortic trunk. The aortic 

 and puhnonic trunks now leave the heart separately. 



The wider dorsal separation of the third and fourth arches is a 

 noteworthy feature in this embryo. The dorsal aorta uniting' them 

 becomes incomplete in the next stage. A distinct short common 

 carotid is present upon either side. 



The presence of short common carotids in this embryo is coin- 

 cident with a change in the place of union of the vessels of the two 

 sides, this being now at the ventral ends of the fourth arches instead 

 of the third pair, as in the preceding stage. 



A comparison of the incomplete vessels in this embryo, between 

 the fourth and sixth arches, with the arteries in this region in the 

 preceding stages, shows that their relation to each other and the 

 vessels which they join is practically in complete correspondence 

 with the condition found in three younger embryos. Since the fifth 

 arch was complete in the stage represented in Fig. 24, Plate 25, 

 and does not occur at all in the embryo slightly older than the one 

 shown in Fig. 26, the incomplete condition in the latter embryo 

 may be regarded as due to the advance of degeneration in this arch. 

 The presence of a fifth entodermal pouch in these embryos, between 

 the fourth and sixth arches strengthens the interpretation that a 

 fifth aortic arch exists between the systemic and pulmonary arches. 



The secondary changes in the formation of the pulmonary 

 artery are now in progress (cf. Bremer, 1902j. 



For the first time in the series, the subclavian arteries arise 

 in front of the union of the aortic roots. 



Embryo of a later stage. Fig. 27, Plate 22, shows the 

 external anatomy of this embryo. I am unable to estimate its age 

 with certainty but it is possibly of the twenty-third day of deve- 

 lopment. The region of the branchial pouches is considerably modi- 

 fied , and only the outline of the first branchial bars can be seen. 

 The back of the embryo has straightened noticeably, and the limbs 

 are well advanced in development. 



Fig. 28, Plate 25, shows a reconstruction of the aortic arches 

 of the right side, excluding the anterior branches of the carotids. 

 The aortic arches now lie considerably further backward than in 

 the preceding stage. 



