GILL-FILAMENTS IN SAUROPSIDA 225 



In his description of embryo ''Robert Meyer No. 335," the 

 same author ('11) figures a section of the "gill rudiment" which 

 he describes as follows: 



Das Relief der Gegend der ersten Tasche wird hauptsachlich von 

 der erwahnten, in das Lumen des Vorderdarmes vorragenden Einstiil- 

 pung beherrscht; sie mag vorlaufig als Kiemenrudiment^ bezeichnet wer- 

 den. Das Kiemenrudiment liegt jederseits ventral und zmn Teil kranial 

 von der Berlihrungsstelle der Epithelien, kaudal vom ersten Aorten- 

 bogen und ragt zapf enformig dorsal und kaudalwarts in das Lumen der 

 Darmbucht vor. In seinem Inneren findet sich ein mesodermaler 

 Kern, Gefasse sind aber in diesem nicht mit voUen Sicherheit nachzu- 

 weisen. 



There are at least three difficulties in the way of accepting 

 Grosser's interpretation, the first of which involves the ento- 

 dermal origin of the structure he presents as a gill filament. 

 Kingsley states in his Comparative Morphology, that the gills of 

 vertebrates 'Svere long regarded as of entodermal origin but in 

 recent years considerable doubt has been thrown on this; at 

 least for fishes, and there is some evidence for their ectodermal 

 origin." Ekman has shown that the ectoderm alone can produce 

 abortive filaments in frogs and toads, while the evidence of the 

 present paper establishes the fact that in the Sauropsida the 

 filamentous structures w^hich have been described as vestigial 

 gills are wholly ectodermal. Another factor unfavorable to 

 Grosser's interpretation is the position of this structure in the 

 auditory pouch. If gills have persisted at all in so highly de- 

 veloped an animal as man, it is not likely that they w^ould persist 

 on arches which least commonly possess gills in water-breathing 

 vertebrates. For with the exception of a few cyclostomes and 

 fishes, gills are never found in the hyomandibular cleft. Again, 

 the tune of development is against Grosser's interpretation. • The 

 inpocketing which he describes first appears at a time when only 

 the first two entodermal pouches have been formed (Embryo R. 

 M. 335, 1.73 mm., 9 to 10 somites) and when the second has not 

 yet reached the ectoderm. It is last met with in embryos no 

 older than R. M. 300 (2.5 n m., 23 somites) where only the first 

 three pouches have reached the ectoderm. In fishes the func- 

 tional gills are never formed before all the clefts have broken 



