46 BULLETIN OF THE 



and hence essentially unpaired, — a view which Hoffmann in Bronn's 

 Thierreich, Eeptilia, unfortunately adopts. 



Van Bemmelen ('88, pp. 114, 115) combats the view of Fritsch, and 

 brings forward additional instances of the persistence of the right proxi- 

 mal root of the a. subvertebralis in embryos. At the same time he 

 points out that Eathke's a. subvertebralis must be regarded as equiva- 

 lent to the dorsal collecting trunks of the anterior branchial vessels, and 

 therefore to the aa. carotides internee of Eathke's general scheme. The 

 vessels designated in my description above by the numeral 2 — called 

 by Eathke a. collateralis colli — are, on this hypothesis, to be regarded 

 as aa. carotides externae (ventral distributing trunks of anterior bran- 

 chial vessels). 



Mackay ('89, pp. 126-136) more tlian any other has contributed by 

 his embryological and comparative anatomical studies to an interpreta- 

 tion of the carotids of the Crocodilia. The vessels a and b are, accord- 

 ing to him, the parts of the ventral distributing trunks which lie between 

 the fourth and third arches, and correspond to the common carotids of 

 Eathke's general scheme. The morphologically paired vessels (1) are the 

 combined internal, or, better, " dorsal " carotids. The part designated in 

 Figure 1 by 1* is thus homologous with the third visceral (first branchial) 

 vessel. The part designated by P -|- P' has arisen by fusion of the dor- 

 sal collecting trunks of the three anterior branchial vessels through a 

 part of their extent. The vessels rnaj-ked 2 and 2' (Fig. 1) are external 

 or " ventral " carotids, — these vessels being represented in Birds also, 

 ■where the so called common carotids are in reality " dorsal " carotids, 

 not equivalent to the common carotids of Lizards. Mackay's results, 

 which thus confirm and extend van Bemmelen's, seem conclusive, not 

 only because he has traced the development of the homologous vessels 

 in Birds, but because he has found one instance — like that of Brandt 

 ('72, p. 307) long ago — in which the dorsal collecting trunk persists 

 between the third and fourth arches. It is connected with the third 

 arch near tlie proximal (posterior) end of the a. subvertebralis, and is 

 thus f\^r removed from Eathke's so called " a. carotis interna " of the 

 head region. The hypothesis that the a. collateralis colli of Crocodiles 

 is homologous with the a. carotis externa of Lizards, receives additional 

 support from the fact that, as in Birds, the a. subclavia arises near the 

 point at which the a. collateralis colli is formed by the division of the 

 true a. carotis communis (a and 1). 



The conclusions of Mackay have been recently confirmed by the re- 

 searches of Hochstetter ('90). 



