l6o ROSS G. HARRISON. 



The experiments are too few in number to warrant very 

 definite conclusions. Absence of gills from both sides seems, 

 in some cases at least, to lead to weakened heart action and 

 ultimate stoppage of the circulation. On the other hand, the 

 last case shows that it is possible for the embryo to adjust itself 

 to the almost complete loss of its respiratory organs, sufficient 

 aeration of the blood probably taking place through the general 

 surface of the body. 



CHANGE OF ORIENTATION OF THE BRANCHIAL ECTODERM. 



These experiments were for the purpose of determining 

 whether there is a definite polarization of the ectodermal elements, 

 and, further, whether any specific gill pattern is predetermined 

 in the region. The experiments of the preceding sections 

 already indicate that the latter is not the'case. 



Transplantations were made in four ways, as was done in the 

 case of the experiments with limbs. The ectoderm was removed 

 from the gill region and replaced by similar ectoderm either 

 from the same or from the opposite side of the body and placed 

 either with the dorsal or with the ventral border of the graft 

 corresponding to the dorsal border of the wound. Besides these 

 a few experiments were made in which the graft was rotated only 

 90. The results may be stated very simply, for there is a marked 

 tendency toward normal development in all combinations. Forty- 

 eight experiments were made, twenty-eight of which gave definite 

 results. Thirteen yielded normal gill-complexes, and these were 

 distributed amongst all of the six different orientations tried. 

 In five cases the second and third gills were normal but the first 

 was small; in two cases the first gill was rudimentary while 

 the others were normal; and in three cases the first gill was 

 altogether lacking, the other two being normal. There were 

 only three cases in which the gill-complex was irregular. 



From these experiments the conclusion may be drawn that 

 the branchial ectoderm is equipotential, and that there is no 

 polarization of the elements affecting the potencies of differ- 

 entiation. In this respect the branchial ectoderm of Amblystoma 

 is markedly different from that of the anurans studied bv Ekman. 



