1 66 ROSS G. HARRISON. 



TRANSPLANTATION OF BRANCHIAL ECTODERM AND MESODERM. 



The results of this experiment are scarcely more definite than 

 when the branchial ectoderm alone is transplanted. There is 

 an initiation of development but it soon becomes arrested. 



In five cases the whole of the branchial mound, including 

 both ectoderm and mesoderm but no endoderm, was placed 

 directly behind the normal gills, i.e., in the region of or just 

 ventral to the pronephros, the ectoderm and mesoderm having 

 been previously removed. About the fourth or fifth day a 

 marked prominence was present in the region of the graft, and, 

 in two of the cases at least, distinct gill sprouts showed. There 

 was, however, no furthei development; the grafted tissue grad- 

 ually flattened out and became reduced to a slight hump or 

 nodule. 



In three other cases the graft was placed in front of the normal 

 gills, in place of the material normally constituting the mandib- 

 ular and hyoid arches. The results were not very different, 

 except that the original gills of the embryo were to some extent 

 disturbed in their development. No gills ever developed in 

 front of the normal ones and in all three cases the balancer was 

 suppressed. 



EFFECT OF LACK OF FUNCTION ON THE DEVELOPMENT OF GILLS. 



It often occurs in experiments with amphibian embryos that 

 the circulatory system does not function properly. Sometimes 

 no movement of the blood can be observed even though the 

 heart does beat. The effect of this upon the gills is noticeable; 

 they never expand fully, and, while preserving their essential 

 character, they have an atrophic appearance. 



In experiments on the gills in particular it not infrequently 

 happens that, although the embryo is otherwise normal, the 

 circulation in the operated gills is either delayed or fails altogether 

 to become established. The same atrophic appearance is found 

 in such cases. 



When the heart is removed a similar condition ensues as 

 regards the gills, but the effects on the embryo as a whole are 

 much more pronounced. It becomes badly swollen and ulti- 



