DEVELOPMENT OF GILLS IN AMPHIBIAN EMBRYO. l6l 



In the latter changes in orientation of this layer are followed by 

 corresponding changes in the position of the gills. This lack 

 of axial differentiation in the gill ectoderm of the Amblystoma 

 embryo lasts, however, for a brief period only, for in slightly 

 later stages (from stage 29 on) change in orientation of the 

 ectoderm is followed by marked disturbances in the arrangement 

 of the gills. 



REMOVAL OF THE MESODERM OF THE BRANCHIAL REGION. 



No satisfactory method of removing the mesoderm of the gill 

 region and replacing it by other mesoderm has been worked out. 

 However, it is a simple matter to remove all of this layer from 

 the region in question and to cover the wound with clean ecto- 

 derm from the branchial region of another embryo. This 

 affords the surrounding mesoderm an opportunity to wander 

 in and fill the place taken by the original branchial mesoderm. 

 The wound after such operations heals, though not nearly so 

 readily as when there is mesoderm underlying the outei layer. 



Sixteen such experiments were made, of which ten were 

 successful. In only two of these cases was there any radical 

 disturbance of the normal development of the gills. The other 

 eight individuals developed approximately normally, though in 

 five of them the heart was affected and the circulation was 

 never established in the embryo. In three cases, however, the 

 circulation was established on both sides, the gills of the operated 

 side remaining smaller than the normal ones. In one the first 

 gill was suppressed. 



From the fact that in such a large proportion of cases the 

 development of the circulation was radically inteifered with, 

 it seems likely that the removal of the mesoderm must have 

 disturbed the material that goes to make up the heart. Aside 

 from this the effect of the operation is slight. The mesoderm 

 of the surrounding region obviously has the potency to fill in 

 the gap made by the removal of tissue and then to take part in 

 the development of normally formed, though at first smalle:, 

 gills. 



