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Scientific Proceedings (131) 



ress of accelerated metamorphosis depends largely upon the 

 ability of the lymphocyte to supply the erythrocytes and leuko- 

 cytes needed. Metamorphic stasis and death we interpret to 

 mean the failure of the lymphocyte to meet the extraordinarily 

 great hemopoietic demands set up by the chain of reactions fol- 

 lowing repeated thyroid administration. This interpretation 

 agrees with our observations on adult frogs following experi- 

 mental hemorrhage and injection of a hemolytic toxin. 



In a recent series of experiments on thyroid-treated tadpoles 

 Swingle 1 has attempted to carry them over the stage of meta- 

 morphic stasis by means of hypophysis grafts. He finds the 

 response variable. In some cases this treatment aids the progress 

 of metamorphosis, in others it does not. We suggest that this 

 variable response may be correlated with the hemopoietic reserve 

 of the animals. In those in which the lymphocytes are still pres- 

 ent in sufficient numbers the hypophysis graft may serve to aid 

 metamorphosis. In those in which the lymphocytes have been 

 practically exhausted the graft may perhaps have no effect. 



Microscopic examination of several regions of a tadpole in 

 process of metamorphosis reveals that there is a definite differ- 

 ential distribution of leukocytes. In the tail, a region of regres- 

 sive change, the predominating types of leukocytes are lymphoid 

 phagocytes (phagocytes derived from lymphocytes) and special 

 granulocytes. Lymphocytes and eosinophils occur only in small 

 numbers. In the intestine, also a region of regressive change, 

 the predominating types are eosinophils and lymphoid phago- 

 cytes. The chief function of the eosinophils and special granu- 

 locytes in these regions appears to be to give off substances which 

 help to break down adjacent tissues, thus rendering these more 

 easy of ingestion and phagocytosis by the phagocytic leukocytes 

 (lymphoid phagocytes chiefly). The eosinophils and special 

 granulocytes themselves often go to pieces in the process and 

 may then be ingested by the phagocytes. 



In the rapidly growing limbs, regions of progressive change, 

 the predominant type of leukocyte is the lymphocyte. Some 

 lymphocytes are brought in by the blood-vessels, but the great 

 majority are differentiated in situ from the mesenchyme. A 

 conspicuous lymphocyte layer is present immediately beneath 

 the basement membrane of the epidermis. Eosinophils, special 



i Journ. Exp. Zool. f 1923, xxxvii, 219-257. 



