404 EMBRYOLOGY OF THE LOWER VERTEBRATES oh. 



end which supplies the Thyroid gland (O'Donoghue, 1912). The 

 blood-supply of the head-region therefore passes to it entirely by 

 the persisting left common carotid (c.c). In Snakes other than 

 Tropidonotus considerable variety exists in the condition of the 

 common carotids. Thus amongst the Boidae the two arteries may 

 remain of approximately equal size (Boa) or on the other hand the 

 left may be reduced (Python). 



In Ohelonians and Crocodiles (Fig. 187a, C) the growth in length 

 of the neck takes place in the region in front of arch III so that 

 here it is the portions of the carotid arteries in front of this. level 

 which undergo elongation. In both of these groups an anastomosis 

 forms in the head-region between dorsal and ventral carotids and, 

 correlated with this, the main blood-stream tends to pass to the head 

 by the dorsal carotids, the ventral vessels becoming to a less (Croco- 

 diles) or greater extent (Chelonians) reduced in size (van Bemmelen, 

 1887; Mackay, 1889). In the Crocodiles a still further modification 

 takes place, inasmuch as the two dorsal carotids become for a 

 considerable part of their length fused together into a single vessel, 

 and following upon this aortic arch III of the right side atrophies, so 

 that here as in Tropidonotus, though for a different reason, the main 

 blood-supply of the head comes from the left side. 



In the Birds the condition closely resembles that of the Crocodile. 

 Here also the ventral carotids become reduced — in this case to the 

 point of complete disappearance — in the neck region as a consequence 

 of an anastomosis with the dorsal carotids in the head. . Here also 

 the enlarged dorsal carotids approach one another on the ventral 

 side of the vertebral column. In those birds which depart least 

 from the primitive condition in this respect (Ostrich, Emu, Casuari, 

 Tinamus, Penguins, Divers, G-ulls, Plovers, Snipe, Rails and their 

 allies, Fowls, Pigeons, Ducks, Ibises, Storks, Herons, Cormorants and 

 some Gannets, Birds of Prey, Parrots, Hornbills, Motmots, Goat- 

 suckers) the two definitive carotids merely lie in proximity to one 

 another. In many birds however they become fused together into a 

 single vessel over a great part of their length and in such a case 

 there may be no further modification (certain Herons such as the 

 common Bittern, some Cockatoos, some Gannets), or, as is the general 

 rule, this fusion is followed by the disappearance of the third aortic 

 arch of the right side just as was the case in the Crocodiles (Rhea, 

 Apteryx, Grebes, Quails, some Cockatoos, Capitonidae, Toucans, 

 Hoopoe, Meropidae, Trogons, Woodpeckers, most Swifts, Humming 

 birds and Passerine birds). In a few cases on the other hand it is 

 the third aortic arch of the left side which becomes reduced to a 

 small vestige (Flamingo) or disappears entirely (Hupodotis — the 

 African Bustard). 



Amongst the'anamniotic Vertebrates typical dorsal and ventral 

 carotids are present in Amphibians and Lung-fishes. The arrange- 

 ment in Urodeles is illustrated by Fig. 186, B : the chief peculiarity 

 to be noted is that the posterior portion of the external carotid (y.c) 



