528 PROCEEDINGS OF THE NATIONAL MUSEUM vol. i04 



aortic arch system and associated vessels with respect to their ulti- 

 mate fate in birds. 



It must be recognized that much of the present interpretation 

 cannot be entnely resolved without further and extensive embryo- 

 logical studies on the nature of the origin and development of these 

 vessels in the various orders, families, and species of birds. The 

 complexities arising from important differences in the final arrange- 

 ment-patterns of the arteries in the neck and thorax add considerably 

 to the difficulties of interpretation. As a result, much of the inter- 

 pretation will of necessity be quite generalized. Furthermore, this 

 interpretation is based largely upon the studies made on the chick 

 embryo, and since there probably are a great many important differ- 

 ences to be encountered in other orders of birds, the present inter- 

 pretations may not be entirely accurate in at least some of the details. 



It is suggested that renewed efforts be made to carry out embryo- 

 logical studies on the development of birds other than the chick, and 

 that the development of the aortic arch system be given especial 

 attention. Among the more critical aspects of this study are the 

 manner and time of fusion of the anterior dorsal radices aortae (dorsal 

 carotids), the manner in which the proximal portion of the dorsal 

 radix, anterior to the carotid arch, atrophies, and the changes in and 

 the fate of the ventral radices aortae (ventral carotids). 



Another factor which should be taken into consideration is that of 

 interpretation of the diagrammatic representations of structure, 

 especially since there are apparent changes in the spatial relationships 

 of portions of the aortic arch system in the different vertebrate groups, 

 and these changes may be brought about as a result of other struc- 

 tural changes or modifications. Some of these structural modifica- 

 tions appear to produce an anterior-posterior compression or contrac- 

 tion of the aortic arch system with corresponding changes in the 

 definitive spatial relationships of the early embryonic system. In 

 the amniotes, and especially in bii'ds and mammals, the ventral aorta 

 appears to be lost, and the ventral radices aortae or ventral carotids 

 are greatly modified. Such a modification in the structural-spatial 

 relationship is rather advanced in the human embryo, with the result 

 that interpretation of true homologies is sometimes very diflBcult. 



Too frequently schematic or diagrammatic representations are or 

 may be misleading as a result of faulty interpretation of both the 

 diagrams and the actual condition as critically observed in study 

 materials. Interpretation of the materials under study, however, 

 should be facilitated by a careful study and analysis of the schematic 

 diagrams. When this has been done, barring the lack of important 

 embryological facts, there should be little difficulty in making ade- 

 quate and correct interpretations. 



As an aid in the interpretation of the adult avian aortic arch deriva- 

 tives, it is well to make comparisons with the aortic arch derivatives 



