1 62 ROSS G. HARRISON. 



CHANGE OF ORIENTATION OF THE BRANCHIAL ECTODERM 



AND MESODERM. 



These experiments are similar to those in which the ectoderm 

 alone was turned, differing only in the circumstance that both 

 ectoderm and mesoderm were lifted and implanted together. 

 There is no difficulty in separating the mesoderm from the 

 endoderm, though the healing of the wounds after transplantation 

 is much less regular than when the ectoderm alone is taken. 

 Inasmuch as turning the ectoderm alone has no effect upon the 

 development of the gills, the effect of the transplantation of the 

 two layers together must be due to the mesoderm alone. The 

 respective results are strikingly different after the two operations. 



There were sixty-three experiments in which both mesoderm 

 and ectoderm were transplanted. Thirty-seven were positive. 

 These cases were distributed unevenly amongst the different 

 orientations, but the results throughout, except when the graft 

 was from the same side of the body and normally oriented, 

 were extremely variable and difficult to place in distinct cate- 

 gories. 



The normally oriented grafts (homopleural dorsodorsal) gave 

 five normal or nearly normal cases and one distinctly abnormal 

 individual in which two gills were irregular and fused together. 



In none of the other orientations were there any absolutely 

 normal cases. However, in the inverted grafts from the opposite 

 side of the body (heteropleural dorsoventral) there were three 

 approximately normal individuals out of eleven positive experi- 

 ments. Of the remaining eight cases, no two have been given 

 the same designation except two which have been classified as 

 very irregular and two others put down as rudimentary. Three 

 gills were distinguishable in five of the eight cases, and only one 

 gill was present in one. In one of the eight cases, and possibly 

 in others, the abnormalities consisted in defects which might 

 have been due to general conditions rather than to any particular 

 disturbance of orientation. 



Inverted grafts from the same side of the body (homopleural 

 dorsoventral) gave not a single case, in a total of thirteen, that 

 even approached the normal condition. Two gills were de- 



