DEVELOPMENT OF GILLS IN AMPHIBIAN EMBRYO. 157 



mation of gills is suppressed. (3) That the external gills and the 

 outer branchial grooves are specifically determined in the ecto- 

 derm and that the endoderm plays no part in the localization of 

 the gills. (4) That the ectodermal rudiments of the gills have, 

 from early stages on, a certain power of self-differentiation which 

 varies amongst the several species studied. (5) That gills which 

 arise from displaced ectoderm, together with atypical. endoderm, 

 conform entirely as to their position, form, and size, to the ecto- 

 derm. (6) That the branchial blood vessels are an essential factor 

 in the later development of the gills. 



The present experiments were made upon the embryo of a 

 urodele, Amblystoma punctatum. They concern almost entirely 

 the two outer layers, ectoderm and mesoderm. Owing to tech- 

 nical difficulties, the endoderm was experimented with only to a 

 very limited extent, and this layer, therefore, does not figure in 

 the present analysis. While certain things are quite definite, 

 the work must be considered as incomplete, inasmuch as there 

 has been no opportunity as yet to examine the specimens in 

 sections. 



With regard to the technique of operation, only one thing 

 requires special mention. The donor embryo was in most cases 

 stained intra vitam in Nile blue sulphate, which colors the ecto- 

 derm bright blue. 3 The stain persists for many days, so that 

 the exact delimitation of the graft can be observed, as develop- 

 ment proceeds. This removes a serious source of error, for it 

 is thus always possible to determine whether the parts under 

 observation are derived from the host or the graft. Embryos 

 from stage 21 (Fig. I, medullary folds just closed) to stage 25 

 (just before appearance of tail bud 4 ) were used for nearly all the 

 experiments. A few were made with earlier and a few with 

 slightly later stages. Subsequent experiments made with still later 

 stages show that marked changes in the behavior of the ectoderm 

 after transplantation occur at about the time when the mesecto- 

 derm from the ganglion crest grows down and the first branchial 

 pouch is developed (stage 29). These will not be considered at 

 present. 



3 Detwiler ('17). 



4 Stages 25 and 29 have been figured previously (Harrison '18, Figs, i and 2). 



