FEEDING THYMUS TO FROG LARV^. 127 



animals of the two groups, practically no differences were 

 observed. The forelimbs of the larvae had not, as yet, broken 

 through the skin, but showed as slight bulges or swellings under 

 the skin of the pectoral region. The growth of all the larvae 

 had been retarded, no doubt, by unseasonably chilly weather. 



June 10, all of the larvse of both thymus and liver-fed cultures, 

 on account of their increased size, were removed from their 

 containers and placed in very much larger basins. The new 

 containers were especially constructed sinks, measuring 30 X 18 X 

 6 inches. Thirty larvae were placed in each sink, which were so 

 arranged that fresh, well-oxygenated water flowed through them 

 constantly. 



By June 20, two of the control larvae, and one of the thymus. 

 fed group, were in advanced stages of metamorphosis. The 

 forelimbs appeared and the tail was reabsorbed. June 26, these 

 larvae had completely metamorphosed. From this date on, 

 metamorphic changes appeared in almost all of the larvae, both 

 of the control and thymus-fed groups. By July 3, six thymus- 

 fed and eight liver-fed larvae had metamorphosed, and by July 

 12, most of the larvae had already metamorphosed, or else were 

 in advanced stages of the process. 



The animals were all killed, and their gonads preserved for 

 microscopic examination. Macroscopic examination of the germ 

 glands of both thymus-fed and control animals revealed no 

 observable differences either as regards the size of the gland or 

 proportion of the sexes. 



The gonads were fixed in Flemming's fluid and sectioned at a 

 thickness of 7.5 fj,. The sections were stained with iron alum 

 haematoxylin, counter stained with congo red. Only four of the 

 gonads were examined microscopically, two from thymus-fed 

 and two from control larvae. 



Microscopic examination of the four gonads revealed nothing 

 of interest. Three of the gonads were from female larvae, and 

 contained young oocytes undergoing growth. The single male 

 gland contained only spermatogonia and was of normal size. 

 The thymus feeding appeared to have had no effect upon the 

 growth and development of the gonads and the germ cells. 



We may say, then, in the light of this experiment, that feeding 



