220 MR. F. E. BEDDARD ON THE [Feb. 19, 



Ductus Ouvieri, attached thereto. The opening of the posterior- 

 cardinal veins into the Ductus Ouviei-i thus undei'goes an altei'a- 

 tion of position with reference to the thoracic vertebrfe. Thus 

 Hochstetter found that in the embryo of a rabbit fifteen days 

 old " Der Stamm der Y. azygos raiindet schon in der Hohe des 

 Zweiten Intercostalraumes in die Hohlvene ihre Seite,"* while 

 in the adult its position is furthei- back. It is interesting to note 

 in relation to such developmental facts the varying position of the 

 entrance of the azygos into the precava in different families and 

 genera of adult Mammals. It would seem plain that if the 

 azygos vein or veins enter the precaval further forward in one form 

 than in another, the conditions ai-e in that type more primitive. 



On the whole it would appear that as a general rule the point 

 of entrance of the azygos vein is opposite or about opposite to 

 the fifth rib. It is at any rate so in Macrojnis^ Lutra, Sciurus, 

 Suricata, Equus, Lemur, (fee. It seems to be the general position 

 among the Oarnivora and Primates. I have not met with any 

 instance of a shifting backwards further than this point, more 

 than the very smallest. I have noticed occasional^ that the 

 sixth rib instead of the fifth is the first to emit an intercostal to 

 the azygos. This was the case, for example, with a new-born Ovis 

 tragelaphus. Now, although Hochstetter and McOlure (working- 

 it is trvie with different types) disagree as to how much exactly 

 of the embryonic postcardinals persist in the adult as the azygos 

 vein or veins, there is no doubt that the actual oi-ifice of the 

 azygos into the precava represents the embouchui-e of the post- 

 cardinal into the Ductus Ouvieri. We can, therefore, argue 

 concerning the shifting position of this point. Dr. McOlure has 

 himself obsei-vedf that the left azygos of Didelphys " opens into 

 the left precava about opposite the head of the third rib," while 

 " the right azygos vein, when present in the adult, opens into 

 the precava about opposite the head of the second rib." It cannot 

 be said, however, that this position of the azygos, which is to be 

 regarded as a more primitive position, is in any way charactei-istic 

 of the Marsupials. It does nevertheless occur in that gi'oup, and 

 I have found the same in Psetodockirus joeret/rwi^fs. It is in- 

 teresting in relation to this matter to find that in the Insectivore 

 Erinaceus the Ductus Ouvieri on both sides of the body lies 

 opposite to the interval between the second and third ribs. The 

 Rodents also show to some extent the same anterior position of 

 the point of entrance of the azj^gos into the precaval. Thus in 

 Hystrix cristata the first affluent of the i-ight azygos arises between 

 the third and fourth ribs. On the other hand, among the Oar- 

 nivora and Primates, as already said, this forward position of the 

 entrance of the azygos is nevei- (in my experience) to be met 

 with. 



Indeed, the facts stated at length in the preceding pages, which 

 have just been briefly reviewed, point distinctly to the persistence 



* Movph. Jahrb. xx. 1893, p. 573. 



f American Jouni. Anat. 1906, p. 185. 



