EXPERIMENTS ON THE DEVELOPMENT OF THE 

 GILLS IN THE AMPHIBIAN EMBRYO. 1 



ROSS G. HARRISON, 

 OSBORN ZOOLOGICAL LABORATORY, YALE UNIVERSITY. 



The present work upon the gills was undertaken as a continu- 

 ation of the study of relations of symmetry in embryonic organs, 

 in the hope that it might throw light on questions raised 

 in previous studies of the limbs. 2 The external gills of the 

 salamander were chosen to this end, as constituting another 

 system with right and left enantiomorphs, each without any 

 plane, axis, or center of symmetry in itself. Owing to the 

 complexity of the system, however, if for no other reason, the 

 experiments failed to give as clear evidence on the questions 

 raised as the limbs had done. Nevertheless, they have brought 

 to light certain interesting facts, which in part corroborate and in 

 part extend those reported by Ekman ('13), who worked upon 

 various anuran embryos. They show, furthermore, certain 

 differences between Amblystoma and the anurans with respect to 

 the organs studied. 



The gill arches and external gills form a complex in which all 

 of the germ layers are represented. There is an outer covering 

 of ectoderm, a central core of mesoderm, which contains the 

 blood vessels and extends out into the gill filaments, and an inner 

 lining to the arches and the clefts consisting of endoderm. 

 Besides these the ganglion crest contributes elements to the 

 arches, chiefly to the formation of the cartilaginous skeleton. 



The experimental work of Ekman upon anurans has shown the 

 following: (i) That when the ectoderm covering the gill region is 

 removed, the regenerated ectoderm is capable of forming gill 

 filaments. (2) That when the gill ectoderm is replaced by 

 ectoderm from another region, with certain exceptions, the for- 



1 Read before the National Academy of Sciences, November 17, 1920 



2 Harrison ('17 and '21). 



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