426 



elusion is dependent upon a series of no less than three assumptions: 

 that the air-bladder is homologous with Miklucho-Maclat's diver- 

 ticulum, that this diverticulum is homologous with that in the bran- 

 chial region of the skate, and that the latter is homologous with the 

 epibranchial groove. The middle member of the series is the weakest. 

 And, of course, the conclusion is opposed to the generally-accepted 

 homology between air-bladder and lung, seeing that in the reptiles, 

 for example, lung and vestigial epibranchial groove both occur. 



After all, we are safe in admitting that a vestige of the epibran- 

 chial groove is present in the embryos of reptiles and birds, of the 

 skate, and perhaps also of the mammalia (Keibel). Another part of 

 the same system, the hypobranchial groove, has assumed a new func- 

 tion; but there is no sure evidence that the epibranchial groove ever 

 did so, and it is passing out of existence. 



Except for the reference to Keibel, nothing has been said of the 

 mammalia. Now Albrecht (1) has drawn attention to the published 

 records of a sac, opening dorsally into the pharynx opposite to the 

 larynx, which is present normally in adult pigs and abnormally in 

 human adults. It has been compared by Klebs with the pouch of 

 Rathke, but by Albrecht with the air-bladder. The latter was 

 trying to prove that the lung has not been derived from the air- 

 bladder, so that his case would have been strengthened had he been 

 able to shaw that the liomologue of the air-bladder was this sac. 

 This paper has been mentioned for the sake of completeness, and 

 without the intention of suggesting that here we have to do with the 

 epibranchial groove. It is difficult to believe that what is confined 

 in the reptiles to their embryos, and is scarcely noticeable in those 

 of birds, should appear in adult mammalia. 



Literature. 



1) Albrecht, P., Sur la non-homologie des poumons des Vertebres 

 pulmones avec la vessie natatoire des Poissons. Paris and Brussels, 

 1886. 



2) Balfour, J. M., A Monograph on the Development of Elasmobranch 

 Fishes, 1878. 



3) — A Treatise on Comparative Embryology, 1881. 



4) Bergfeldt, a., Chordascheiden und Hypochorda bei Alytes obste- 

 tricans. Anat. Hefte, Bd. 7, 1896. 



5) Eisig, H., Der Nebendarm der Capitelliden und seine Homologa. 

 Zool. Anz., Bd. 1, 1878. 



6) — , Monographie der Capitelliden des Golfes vom Neapel. Fauna 

 und Flora des Golfes vom Neapel, Bd. 16, 1887. 



