DEVELOPMENT OF GILLS IX AMPHIBIAN EMBRYO. 167 



mately dies. Operations of this kind were done upon embryos 

 in stages 33 and 34, i.e., just before the heart begins to beat. It 

 is difficult to remove the heart rudiment completely. There is 

 almost always some regeneration, and pulsation is established, 

 which, however, is usually ineffective and in such cases no blood 

 can be seen in movement in the yolk vessels or through the gills. 

 Nevertheless, the latter grow out, and are normally constituted 

 except for their atrophic appearance. The operculum and the 

 gill plates are also formed. As regards the development of 

 their specific form, the gills, like many other structures, are thus 

 independent of function, though the marks of atrophy in the 

 functionless organs are unmistakable. 



CONCLUSION AND SUMMARY. 



So far as the present experimental analysis has led, the respec- 

 tive roles of ectoderm and mesoderm in the formation of the 

 gills are as follows: 



The branchial ectoderm is specifically gill forming, but sur- 

 rounding the gill region, and, more particularly, posterior to it, 

 the ectoderm has the potency to form gills in a diminishing 

 intensity as the. distance from the gills increases. This is shown 

 both by transplantation experiments and by regeneration after 

 removal of the branchial ectoderm. 



The gill pattern is not, however, laid down in the branchial 

 ectoderm, as is shown by the fact that this same ectoderm, when 

 turned in any way or even when taken from the opposite side, 

 gives rise to normal gills notwithstanding. 



The specific pattern must therefore be determined by the 

 deeper layers, but in the absence of experiments upon the endo- 

 derm, the exact roles of mesoderm and endoderm, respectively, 

 cannot be determined. The indications are, however, that the 

 endoderm does not play merely a passive part, as Ekman main- 

 tains to be the case in the anurans. 



The mesoderm cannot be a well-defined mosaic, for doubling 

 of the material by superposition, provided the orientation of the 

 grafted tissue is normal, does not disturb the normal development 

 of the gills. Abnormal orientation of the mesoderm, however, 



