306 INFLUENCE OF THYMUS 



Inhibitory EJfect of the Thymus. 



Some experiments may be reported briefly showing that thymus 

 feeding sometimes actually prevents metamorphosis, though in the 

 majority of the larvae it has no such effect. 



In a set of larvae of Amhy stoma maculatum, kept at approximately 

 15°C., fed on tubifex and earthworms, and consisting of about 100 

 specimens, most of the larvae, which had hatched in the beginning of 

 May, 1916, metamorphosed during the months of August and Sep- 

 tember. On October 21, eight of these were still larvae. These 

 were picked out to form a series of normal controls (c. W. 1916), 

 In a set of eighteen larvae of the same species and of the same age 

 and kept under the same conditions as the above series, but fed on 

 thymus soon after hatching, nine larvae had metamorphosed up 

 to October 21. The remaining nine were picked out to form a 

 thymus-fed series (c. T. 1916). The worm-fed larvas of Series c. W. 

 1916 metamorphosed at an age of 28 weeks and 6 days on an average; 

 the first larva to metamorphose was 25 weeks and 4 days old at the 

 time; the last one 33 weeks and 4 days. Among four larvae of the 

 thymus-fed Series c. T. 1916, the first animal was 27 weeks old when it 

 metamorphosed, the last one 55 weeks and 2 days. Of these four larvae 

 the average age at the time of metamorphosis was 35 weeks and 2 days. 

 The other five larvae of the thymus-fed series died before meta- 

 morphosis; three at the age of 31 weeks and 2 days, one at the age 

 of 40 weeks and 3 days, and one at the age of 62 weeks and 3 days. 

 Thus one animal of the thymus-fed series remained larval for about 

 1 year and 3 months; at this time it did not show any signs of meta- 

 morphosis and it seems possible that it would have remained per- 

 manently in a larval state. The writer so far has never observed in 

 his normal controls individuals which remained larval for so long a 

 time. Thus there can be no doubt that in this thymus-fed series 

 metamorphosis was retarded as compared with the controls and in 

 one case probably was even prevented. 



In a series of eight larvae of Amhy stoma opacum, which were kept at 

 about 25°C. and fed moderately on earthworms, the average time of 

 metamorphosis was 26 weeks; in a series of eight larvae of the same 

 species of the same age and from eggs of the same female as the 



