Leucocytes and Metamorphosis 



381 



here the principal hemopoietic center, the kidney playing little 

 part. At some time in the developmental history of the frog, 

 therefore, the chief locus of blood-cell formation is shifted from 

 the kidney to the spleen. A study has been made to determine the 

 effect upon the hemopoietic centers of accelerated metamorphosis 

 induced by feeding tadpoles with thyroid extract. 



A few days after the first administration of thyroid extract 

 definite changes may be seen in the kidney and spleen. The inter- 

 tubular stroma of the kidney is largely drained of its blood-cells. 

 Some stimulation of the hemopoietic tissue that is left occurs. 

 Erythrocytes, lymphocytes, special granulocytes (pseudo-eosino- 

 phils or neutrophils) and eosinophils are drawn from the kidney 

 in great numbers. A marked shrinkage in size of this organ re- 

 sults which is evident microscopically. At the same time a myeloid 

 metaplasia is effected in the spleen and mesentery. Erythrocytes 

 are drained from the spleen and the splenic lymphocytes stimu- 

 lated to differentiate into other erythrocytes. Lymphocytes from 

 other parts of the body migrate to the spleen and are incorpo- 

 rated in it. The net result of this activity in animals in which 

 metamorphosis is not too greatly accelerated is an increase in 

 the proportion of lymphocytes to erythrocytes, and some in- 

 crease in absolute size of the spleen. In these animals it may be 

 said that the partial drainage of the intertubular regions of the 

 kidney, and the stimulation of the spleen, apparently indicate 

 the initiation of the shift of the main hemopoietic locus from 

 the kidney to the spleen. Whether or not this shift can be car- 

 ried to completion experimentally has not yet been determined. 



Accelerated metamorphosis results in high mortality. All of 

 our thyroid-treated tadpoles have died before complete resorption 

 of the tail. Death usually occurs in this condition of meta- 

 morphic stasis. Examination of the hemopoietic organs of ani- 

 mals that have reached this stage suggests that death comes as 

 a result of anemia. The spleen and kidney show that great num- 

 bers of their blood-cells have been drawn from them, the spleen 

 being often totally exhausted. Accelerated metamorphosis sets 

 up a demand for erythrocytes to furnish the basis for increased 

 oxidation, and for leukocytes to aid in the processes of regres- 

 sive and progressive change. The lymphocyte is the progenitor 

 of the erythrocyte and of various types of leukocytes, and is 

 itself derived from the mesenchymal cell. The successful prog- 



