Leucocytes and Metamorphosis 383 



granulocytes and phagocytes are almost entirely absent. Carrel's 

 recent experiments 2 dealing with the effect of leukocyte ex- 

 tract and leukocyte secretions upon tissue cultures show 

 that leukocytes give off substances that stimulate growth of 

 surrounding cells. Carrel used a mixture of all leukocytes in 

 these experiments. In the rapidly growing tadpole limb, 

 since the lymphocyte is the only type of leukocyte present in 

 any numbers, it is obvious that any growth-promoting leuko- 

 cytic secretion must come from it. Examination of the skin 

 from the abdomen and back of the same tadpoles that have the 

 conspicuous lymphocyte layer in the limbs shows that there is 

 no similar collection of differentiating lymphocytes in these 

 regions (regions in which little or no growth is taking place). 

 From the negative standpoint, it would seem that eosinophils, 

 special granulocytes and phagocytes could be ruled out from 

 having a possible growth-promoting function since they pre- 

 dominate in the tadpole in regions of regressive change. We 

 suggest, therefore, that the growth-promoting function of leuko- 

 cytes, as demonstrated by Carrel, should probably be ascribed to 

 the lymphocyte component of leukocytes. 



In conclusion, it may be pointed out that the leukocytes are 

 an important part of the mechanism by which the thyroid secre- 

 tion brings about its characteristic results. Metamorphic differen- 

 tiation is largely a sequence of progressive and regressive 

 changes in various parts of the body. Lymphocytes are cor- 

 related with the former, granulocytes and phagocytes with the 

 latter. The successful maintenance of the increased oxidation 

 following thyroid administration depends upon the lymphocyte 

 as progenitor of the erythrocyte. 



ZJourn. Exp. Med., 1922, xxvi, 385-391. 



