1 64 ROSS G. HARRISON. 



pleural dorsoventral) formed in all five cases gills which ap- 

 proached the normal condition. In two of these the gills were 

 distinctly smaller than normal and in the other three there was 

 considerable fusion and some irregularity, though all three gills 

 could be distinguished. 



On the other hand, the inverted grafts from the same side of 

 the body (homopleural dorsoventral) showed in all five cases a 

 very defective development of the gills, only one small, irregular 

 or abortive gill being formed in each case. 



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

 (heteropleural dorsodorsal) the results were likewise for the 

 most part irregular or defective. Two fused gills developed in 

 two cases, and in the other three only a single small, irregular 

 or abortive gill was formed. 



The results of these experiments show that while the doubling 

 of the mesodermal material has in itself little or no morphogenetic 

 effect, the orientation of the superadded material is of consider- 

 able consequence in determining whether aberrations in the 

 development of the gills occur or not. Aside from experiments 

 in which the transplanted material is normally oriented, these 

 disturbances are apparently least marked in the case of inverted 

 grafts from the opposite side of the body, and in this respect 

 there is an approach to the condition found in the mesoderm of the 

 fore limb bud. However, the results of this orientation of the 

 grafted gill mesoderm are not normal with sufficient constancy 

 to term the combination "harmonic", as was done in the case 

 of the limb. 



REMOVAL OF BOTH ECTODERM AND MESODERM WITHOUT 

 REPLACEMENT OF EITHER. 



This experiment is a by-product of the experiments described 

 in the last two sections. The individuals are the donor embryos 

 of those operations. 



With such a large deep wound as is necessary in removing 

 the gill mesoderm, a high mortality was to have been expected. 

 Out of one hundred and six operations, only ten cases survived 

 ten days or longer. Five others were preserved at intervals 

 from an age of two to that of eight days. 



