which case its asymmetry is reversed (figs. 36 to 39), just as in the corresponding class of heterotopic transplantations, or it gradually rotates towards its normal position while retaining its original prospective asymmetry (figs. 56 to 59). In the former alternative duplicate limbs are nearly always formed. Only in the one case, referred to above, did a perfect single limb arise. In the other alternative single limbs usually arise, though some of the cases of reduplication certainly belong to this group.

In the duplicities belonging to the first group the original limb bud continues to grow in an anterior direction and ultimately becomes a reversed limb. After a time a reduplicating bud appears on the posterior border of the original bud (fig. 44) and in the clearer cases grows into a homopleural limb in approximately normal posture (figs. 45 to 48). The original bud becomes a reversed limb which, together with the reduplicating member, may form an almost symmetrical complex.

Twenty-four of the thirty-one36 cases of reduplicated limbs are probably of this type. Fifteen are certainly so,37 and in three others that are very similar all that is lacking to place them unequivocally in this group is a definite observation as to which bud was the primary one; six more cases39 may also be interpreted in the same manner, though they are not sufficiently clear to insure that this is the only possible interpretation.

The degree of reduplication varies here, as in the other groups of experiments, from the condition where almost the whole arm is involved to that in which the hand is only partly double. In three cases the anterior bud was much reduced (p. 49), the posterior bud becoming a somewhat irregular homopleural limb. In eleven cases there is only one reduplicating appendage, which is always posterior to the primary (figs. 43 to 48), while in the remaining twelve there are evidences of further doubling, usu

36 Four cases are considered here which are not included in the tabulation on account of the fact that the wound was only 3 somites in diameter (I. E. 39, 41, 44 and 45).

37 I. E. 48, 60, 62, 63, 66, 72, 74, 75, 81, 85, 87, 89, 91, 92, and 96.

38 I. E. 44, 45, and 52.

39 I. E. 39, 68, 70, 93, 100, 102.

40 I. E. 92, 93, 100.

41 I. E. 39, 45, 62, 63, 66, 72, 75, 81, 85, 87, 91, and 93.

ally on the anterior side of the original (figs. 49 to 55 and 61). When the latter condition arises and the anterior reduplicating member is sufficiently developed, it is seen that it, too, is mirrored from the original member and is homopleural (fig. 55). In one case there are three complete hands, one of which has two of its digits doubled.42 The plane from which the posterior reduplicating member is mirrored in the final form of the limb varies from radial to dorsal (figs. 3 and 4) and is usually intermediate between these two extremes (p. 13). Nineteen cases follow this rule, three are indeterminate and there is only one positive exceptional case, in which the mirror plane is ulnodorsal.43 When there is also an anterior reduplicating member, it is generally mirrored from a plane 180° around the limb axis from the first; i.e., ulnar, ulnopalmar, or palmar.

The reduplications belonging to the second group are more restricted and less certain of diagnosis. The limb bud retains its original prospective asymmetry, reaching an approximately normal position by rotation, and reduplication is much less extensive, involving in most cases the digits only (fig. 62). Three cases44 almost certainly belong to this group, and there may be two others. 45

Of the two remaining cases of reduplication, one died too young; in the second the supernumerary limb was of the same side as the primary and was quite distinct from it. This is a very unusual condition, but the transplanted bud in this case was

Figs. 49 to 55 Orthotopic transplantation; left limb bud to left side inverted (hom.dv.), resulting in limb with two reduplicating members. Exp. I. E. 63. N, normal right limb bud; TR, transplanted left bud; PR, primary limb; A.DU, anterior, and P. DU, posterior reduplicating members. X 10.

Fig. 49 Dorsal view, four days after operation.

Fig. 55 Dorsal view of specimen preserved seventeen days after operation. Gills (BR) removed to show limb. 1 to 3, numbers of digits.

42 I. E. 87. 43 I. E. 72. 44 I. E. 86, 88, 90. 45 I. E. 41, 59.

46 I. E. 38.

larger than usual (four somites in diameter), and it is possible that the reduplicating bud, growing from near its anterior border, was uninfluenced by the primary limb and hence was not mirrored.

There remain for consideration those cases in which a single non-reversed limb developed. As in the other cases, the limb bud in these showed at first the consequences of abnormal orientation. When first observed it pointed more sharply laterally and more dorsally (less posteriorly) than normal. Two even pointed dorsally and slightly anteriorly. In the course of development the limb gradually changed its posture and ultimately came to a perfectly normal posture by a process of rotation at the shoulder-joint (figs. 56 to 59). Ten such cases were obtained, 47 though in several of them48 it is possible that reversal may have been brought about by early reduplication and suppression of the original bud, as described in the next section (p. 49). In one of these19 a supernumerary radial digit was present, but this is to be regarded as a case of hyperdactyly rather than one of mirrored reduplication. In one cases the limb which originally developed showed irregularities in the digits. The arm was then amputated above the elbow, and the appendage which regenerated was in every respect normal. This case is of considerable interest in showing that the abnormal condition which produces reduplication is not necessarily stamped upon the whole structure, but may be due to some local mechanical disturbance.

In reviewing this group of experiments, it is clear that the first two results, i.e., single reversed limbs and most of the reduplications, come under the same scheme. There is a primary reversal of asymmetry, without reduplication in the first case and accom

Figs. 56 to 59 Orthotopic transplantation; left limb bud inverted (hom.dv.). Limb reaches normal posture by rotation. Exp. I. E. 49. N, normal (right) limb; TR, transplanted (left) limb.

Fig. 56 Eleven days after operation.

Fig. 57 Twenty-one days after operation.

Fig. 58 Thirty-eight days after operation.

Fig. 59 Preserved specimen, killed at thirty-nine days.

47 I. E. 49, 55, 69, 71, 73, 77, 84, 94, 99, and 101. 48 For instance, I. E. 77.

49 I. E. 101.

50 I. E. 71.