AORTIC ARCHES OF BIRDS — GLENNY 535 



may fuse with the radix and be completely lost or remain as a linea 

 botalli, or it may be partially resorbed and remain as an incomplete 

 ligament or as a ligamentous button on the ventral surface of the radix 

 aortae. 



Atrophy of the right ductus botalli and the left radix aortae occurs 

 at approximately the same time and at about the same rate. It 

 appears that, as in many species of birds, there may be a continued 

 progressive atrophy of both of these structures for quite a time after 

 hatching. The rate and level of atrophy of these structures may 

 differ in different species, but particularly between families and orders 

 of birds. It appears that, in a few orders and families of birds, atrophy 

 of these two structures may be independent of each other. This 

 assumption is based on observations on many species within a family 

 or order in which the ligamentum aortae may be of considerable size, 

 while the right ligamentum botalli is almost entirely or completely 

 lacking or remains as a linea botalli. 



Much confusion and misunderstanding is encountered in the litera- 

 ture with respect to the carotid arteries. This is in part due to the 

 lack of uniformity in terminology and to the failm'e to recognize some 

 definitive vessels which are embryonic derivatives. Incomplete 

 series for study, along with inadequate techniques, account in part 

 for the failure of earlier workers to fully comprehend the significant 

 changes which occur during the first week or 10 days of incubation. 

 Furthermore, many of the earlier workers probably were greatly 

 influenced in their views and interpretations by the dominant con- 

 cept of ontogenetic recapitulation which so strongly influenced the 

 studies of morphologists during the 19 th century. 



Some authors refer to the dorsal and ventral radices aortae simply 

 as the dorsal and ventral carotids. This may have led to some mis- 

 interpretation, since the internal and external carotids are sometimes 

 referred to as the dorsal and ventral carotids. Interpretation is 

 difficult because direct comparisons cannot be made between birds 

 and reptiles on the one hand or between birds and mammals on the 

 other hand since the development of these vessels differs somewhat in 

 details in each of the three classes of amniotes. 



According to Twining (1906), the third aortic arch gives rise to a 

 dorsal carotid and a ventral carotid; the former is well developed and 

 easfly traced anteriorly, while the latter, which he regards as the 

 basal remnant of the first and second aortic arches, arises from the 

 base of the third arch. At this early stage no trace is found of a vessel 

 connecting the dorsal and ventral carotids, the entire blood supply to 

 the jaw anlagen being produced by the ventral carotids. Anastomosis 

 of the dorsal and ventral carotids occurs during a later stage in the 

 embryonic development. 



Increase in length of the dorsal and ventral carotids results from 

 elongation of the cervical region, and this is followed by many complex 



