158 ROSS G. HARRISON. 



REMOVAL OF THE ECTODERM OF THE BRANCHIAL REGION 



AND ITS REPLACEMENT BY ECTODERM FROM 



ANOTHER PART OF THE BODY. 



The gill region in an embryo with closed medullary folds is 

 shown in outline in Fig. I (BR). If this ectoderm is removed 

 and replaced by ectoderm from the flank (FL) or any other part 

 except that immediately surrounding the gill region, complete 

 suppression of the operculum and the gills on the operated side 

 (Fig. 2) results. Thus, out of twenty-four cases in which ecto- 

 derm from the flank was used, seventeen had the gills entirely 

 lacking. In six cases in which the third gill developed and in 

 one in which the first gill was formed, ectoderm from the host 

 covered the greater part of the gill including the tip. 



When the covering ectoderm is taken from the region just 

 behind the gills, i.e., from the pronephric and limb area (PN), 

 functional gills develop. This was found in five cases, in three 

 of which all three gills were well developed, while in the two 

 others the first gill was either absent or rudimentary. 



When, however, the ectoderm is taken from the ventral mid- 

 region (CD), i.e., from the region immediately ventral to the 

 branchial, the result is not so favorable. Out of nine cases, no 

 gills at all developed in four, a functional third gill in one, a 

 functional third and rudimentary second in three, and two 

 functional gills (second and third) in one. 



Likewise, when the covering is taken from the anterior part 

 of the head, there is usually a paitial development of the gills, 

 involving principally the second and third. The transplanted 

 piece was taken in various ways (Fig. i, C, C f , A). In general, 

 it may be said that the closer the seat of origin of the graft is to 

 the normal branchial region, the more perfect are the gills that 

 develop. A supernumerary balancer developed in almost all of 

 the cases in which the graft included the normal balancer ecto- 

 derm. 



When the ectoderm covering the gill region is removed and 

 the w r ound left uncovered, as it was in the donor embryos in 

 many experiments, the surrounding ectoderm pushes over the 

 wound, covering it usually in twenty-four hours or a little more. 



