192 BENNET M. ALLEN 



MATERIAL AND METHODS 



Muller (71), de Meuron ('86), and Maurer ('87) have given 

 accounts of the origin of the thymus glands in the anurans. 

 According to these authors, they arise as a pair of epithehal buds 

 from the dorsal portion of the second gill pouches. This fact 

 makes the anurans especially favorable for experiments in extir- 

 pation of these organs during larval life. In this series of exper- 

 iments the larvae of Rana pipiens were used. The early thymus 

 buds were removed with a pair of needles at the stage when the 

 intestines were beginning to twist — 8 to 8.5 mm. total length. 

 One of the needles is broken off and ground to a knife edge. 

 After narcotizing the tadpoles with chloretone, the knife-edged 

 needle is inserted into the side of the head just beneath and be- 

 hind the auditory vesicle and the cut is carried forward extending 

 to a point beneath the eye. This usually causes considerable 

 loss of blood, owing to the cutting of the anterior cardinal vein. 

 There is a slight initial retardation of development probably re- 

 sulting from this. But such effects are of short duration. Both 

 needles are now used to spread open the wound. Any blood 

 clots are removed, and the white spherical thymus bud is now 

 seen as shown in the figure. The knife-edged needle is used to 

 cut away this bud together with the adjacent portion of the 

 second gill arch to which it is attached. Both needles must often 

 be used to grasp and pick away the thymus bud. In each case 

 the bud was carefully examined and identified after removal. 

 Of course, it is necessary to operate on both sides in this manner. 

 This necessitates making two separate incisions. The tadpoles 

 rapidly recover and undergo normal development, passing 

 through metamorphosis in the usual time and in normal fashion. 



This experiment was performed in the spring of 1917 and 1918. 

 The first season six of the operated tadpoles metamorphosed at 

 the date indicated in the table. One other was somewhat under- 

 nourished and did not transform until the following spring when 

 it was found to be of full size — rather above the ordinary. This, 

 however, was the only case of retarded development and is in 

 no wise different from similar cases observed among controls. 



