DEVELOPMENT OF GILLS IN AMPHIBIAN EMBRYO. 159 



The mortality among these embryos is high, but when they live, 

 gills nearly always develop from this regenerated ectoderm. In 

 some cases the gills formed are normal; in others, there is a 

 reduction in number or in size or both, the first gill being more 

 often affected than the others. 



BILATERAL EXTIRPATION OF BRANCHIAL ECTODERM. 



As far as the question of the morphology of the gills is con- 

 cerned, this experiment is in no wise different from the unilateral 

 operations, but physiologically it may prove of greater interest. 

 At present only a few cases are available for study. In three 

 experiments the branchial ectoderm was removed on both sides 

 and replaced by ectoderm from the flank. In two of these 

 cases no gills at all developed, while in one the first gill on the 

 left side, which was partly covered by ectoderm from the host, 

 was rudimentary. In the last-named case the circulation of 

 the blood was never established and the embryo became cedem- 

 atous. In both of the other cases the circulation was estab- 

 lished in the vessels of the yolk, but the heart action became 

 more and more feeble, and after a time the circulation of the 

 blood ceased entirely. Both of these eventually became oedem- 

 atous also, though not extremely so. One individual, killed 

 fourteen days after the operation, showed marked lack of sen- 

 sitiveness to tactile stimuli, and responded to a needle prick 

 only with a single jerk. It was apparently unable to swim. 

 The other could execute swimming movements fairly w r ell, 

 though in a somewhat jerky fashion. A fourth case that should 

 be considered in this connection had the branchial ectoderm 

 removed on both sides after which it was allowed to regenerate 

 without replacement from elsewhere. The operculum was 

 formed on both sides but the gills failed to develop except in 

 most rudimentary form on the right side two rudimentary 

 gills and on the left only a sharp conical process (Fig. 3). Blood 

 circulating in these gills was first observed thirteen days after 

 the operation. The larva was kept under observation for thirty- 

 six days, at which age it was preserved. Its development, 

 apart from the absence of gills, was normal. 



