DEVELOPMENT OF GILLS IN AMPHIBIAN EMBRYO 163 



veloped in five cases, but they were abnormal in some respects 

 (Fig. 4) ; in the others only one gill was formed. Two individuals 

 had quite irregular gills and three had rudimentary ones. 



In the non-inverted grafts from the opposite side of the body 

 (heteropleural dorsodorsal), of which there were only six available 

 cases, the results were in no two cases exactly alike. In one a 

 nearly normal complex developed. In two others three gills 

 developed, but with abnormalities. The others were still less 

 perfect. 



The above results, standing alone, do not lead to any definite 

 conclusion regarding the exact role of the mesoderm in the 

 formation of the gills or regarding the nature of the disturbance 

 caused by abnormal orientation of this layer. It is not clear 

 that it is due simply to the reversal of a single polarized 

 axis, such as is the case in the mesoderm of the anterior limb, 

 although the fact that there is a certain tendency toward normal 

 development in the inverted grafts from the opposite side of 

 the body might be taken to indicate that the anteroposterior 

 axis of the elements of this layer is more markedly polarized 

 than is the dorsoventral. The experiments described in the 

 next section point somewhat more clearly to such a conclusion. 



EFFECT OF INCREASING THE AMOUNT OF MESODERM IN THE 

 BRANCHIAL REGION. SUPERPOSED GRAFTS. 



In this group of experiments the branchial ectoderm was 

 removed without materially injuring the underlying mesoderm, 

 and a piece of tissue of the same size and shape including both 

 mesoderm and ectoderm was healed over the wound. In this 

 way the amount of mesoderm w r as approximately doubled, 

 there having been only a very slight loss of mesoderm cells 

 through the operation. The grafts were oriented in the four 

 different ways as in the previous experiments. 



The group with normal orientation (homopleural dorsodorsal) 

 developed gills that were normal or nearly so in seven cases 

 (Fig. 5), and in only one, in which the three gills were closely 

 fused, was there any marked abnormality. 



The inverted grafts from the opposite side of the body (hetero- 



