344 C - C - SPEIDEL. 



became quite red and vascular and grew rapidly, almost remind- 

 ing one of the effect of thyroid extract. The writer became 

 suspicious of the thymus extract used and had it analyzed for 

 the presence of iodine. 5 The analysis gave negative results. 

 The later history of the tadpoles showed that only growth in 

 size of the larval structures was being stimulated, and not differ- 

 entiation. There was no acceleration in skin degeneration of the 

 forelimb region, except that caused secondarily by pressure of 

 the growing forelimb. There was likewise no reduction in the 

 tail, but on the contrary growth. With thymus treatment limb 

 growth appears to proceed relatively faster than the general 

 process of body differentiation. As a result, the forelimbs en- 

 closed beneath the skin reach a comparative size and strength, 

 such that they become the important factor in determining the 

 time of eruption. With increasing limb size the spiracle becomes 

 less important since the forelimb cannot be pushed through it 

 without complementary skin degeneration. Since size of fore- 

 limb and amount of skin degeneration on the two sides are about 

 equal in the species under observation a more equal ratio of 

 righthandedness to lefthandedness results. 



The writer does not wish here to enter the controversy as to 

 whether or not the thymus extract has a specific endocrine con- 

 tent. Its effect upon symmetry in forelimb eruption in tadpoles 

 seems to be best explained on the grounds given above; i.e., its 

 unquestioned value as a highly nutritious and therefore growth- 

 promoting food. It is not necessary to assume a specific endo- 

 crine effect. This much, however, may be added. Thymus 

 gland is largely lymphoid tissue. In the light of Carrel's work 

 showing the growth-promoting effect of leucocytic secretions or 

 "trephones" ('24), and the confirmatory observations of Jordan 

 and Speidel ('23) on lymphocytes in rapidly growing regions in 

 tadpole metamorphosis, it would seem probable that growth- 

 promoting substances (trephones) of lymphocyte origin would be 

 present in thymus extract. 



In conclusion, it may be pointed out that these observations 

 and their interpretation, though of little importance in them- 

 selves, suggest the possibility of the following principle operating 



6 The analysis was made by Mr. T. F. Otto, of the University of Virginia Med- 

 ical School. 



