70 
ROSS G. HARRISON 
impossibility of grafting successfully pieces with the mesoderm 
turned toward the outside and by the difficulty of handling 
pieces of mesoderm free from ectoderm without disturbing their 
arrangement. Perhaps we may consider ourselves fortunate in 
being subject to such restrictions. 
HOM.DD 
HOM.DV 
HET.DD 
HET.DV 
Fig. 120 Diagram showing the sixteen possible combinations (1 to 16) obtain- 
able by transplanting half limb buds. The shaded area signifies the stationary 
half, the clear area the transplanted half. R, right; L, left;Z), Dorsal; V, ven- 
tral; A, anterior; P, posterior. The operations are represented as on the right 
side of the embryo. 
Returning to the experiments actually carried out (fig. 120), we 
find that four of them consist merely in replacing the excised 
piece with another of exactly the same kind in normal orientation. 
These serve, therefore, as controls for testing the effect of the 
operation as such on the further course of development. It is 
also seen that half of the combinations are harmonic and half dis- 
harmonic (p. 8). Half are of course homogeneous or com- 
