In anura larvae the thymus gland m o ves caudally during the course of development from the place of its first appearance. What causes this migration ? In this respect a number of possibilities can be thought of and these were experimentally studied in this paper.
Experiments were carried out in an early stage of Bufo japonicus and the specimens were examined just at the end of metamorphosis. Firstly is locomotion an intrinsic quality of the gland itself ? In answer to this question the author extirpated the thymus and grafted thyroid gland in its place. Examination afterwards reveals that the thyroid gland changes its position caudally just as the thymus gland does. Such caudal migration can not be therefore specific for the thymus gland. Furthermore if the thymus gland were displaced caudally as regards its normal definitive position, then the gland did not come back afterwards to the normal position. The same is true with cranial displacement of the gland, that is there is no caudal migration to the normal position. These 2 series of experiments clearly show that no interaction such as biotaxis exists between the gland and the tissues. Secondly the mechanical factors must be considered. In normal deve- lopment M. depr. mandibulae develops in front of the gland and it seems in farther development as if it might push the gland backwards to the final position. Though this muscle was removed at an early stage, the gland still takes its normal course of migration. Thus the muscle has no influence upon the migration. The ear vesicle is still small when the thymus appears as a bud and the more the former grows the more the latter moves backwards. In Hertwig's Handbook, it is stated as follows: ‘ Die Volumsentfaltung des Labyrinths drängt die Thymus etwas ventral herab, und unter der Ausbildung der Paukenhöhle ruck sie welter nach hinten. ’ The extirpation of the ear vesicle in an early stage of development does not, however, affect the movement of the gland.
In cases of grafting another thymus besides the normal one it is theoretically possible that only one gland moves and the other remains in its position. But the fact is that both migrate backwards together. On the other hand, when the thymus was artificially severed earlier than normal from the branchial cleft, the gland locomotes normally. Moreover early separation of the bud does not induce regeneration of another thymus. From this findings may be drawn the following conclusions.
1. For migration of the thymus it is essential that this organ lie on its proper position.
2. Migration of the thymus gland is not specific for this organ. (Any other organ can move backwards in place of thymus).
3. The pressure of the M. depr. mandibulae and the ear vesicle has nothing to do with migration of thymus.
4. Consequently we can only conclude that migration of the thymus occurs owing to some difference in rate of growth between the thymusregion and other regions.

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