168 ROSS G. HARRISON. 



both after extirpation of the normal mesoderm and in cases of 

 superposition, results in marked aberration from the normal 

 development, showing that this layer is not without axial dif- 

 ferentiation. These aberrations from the normal are not specific 

 for any particular orientation, but there is a greater degree of 

 disturbance after the orientations which, in the case of the fore 

 limb, have been found to be disharmonic, i.e., inverted mesoderm 

 from the same side of the head and non-inverted from the 

 opposite side. One of the combinations called harmonic in 

 the case of the fore limb (that in which the graft is normally 

 oriented) gives normal gills, but the other, while yielding a 

 considerable number of cases of approximately normal gills, 

 results on the other hand in a good many irregulaiities. These 

 circumstances may be taken to indicate that the gill-mesoderm 

 elements are distinctly polarized in an anteroposterior direction 

 and perhaps less markedly so dorsoventrally. The fact that 

 the mesoderm from the periphery of the gill region is capable 

 of forming gills, taken together with the fact that doubling the 

 amount of the material does not disturb normal development, 

 points to the conclusion that this mesoderm is equipotent. 



REFERENCES. 

 Detwiler, Samuel Randall 



'17 On the Use of Nile Blue Sulphate in Embryonic Tissue Transplantations. 



Anat. Rec., Vol. 13. 

 Ekman, Gunnar 



'13 Experimentelle Untersuchungen iiber die Entwicklung der Kiemenregion 

 (Kiemenfaden und Kiemenspaltenj einiger anuren Amphibien. Morph. 

 Jahrb., Bd. 47. 

 Harrison, Ross G. 



'17 Transplantation of Limbs. Proc. Nat. Acad. ScL, Vol. 3. 



'18 Experiments on the Development of the Fore Limb of Amblystoma, a 



Self-Differentiating Equipotential System. Jour. Exp. Zool., Vol. 25.. 

 '21 On Relations of Symmetry in Transplanted Limbs. Jour. Exp. Zool., 

 Vol. 32. 



