ENDOCRINE GLANDS AND BILATERAL SYMMETRY. 337 



tions represented by the thyroid and thymus extracts actually 

 affect in a differential manner the bilateral symmetry of the 

 developing animals. Further experiments were set under way 

 in an attempt to analyze the factors controlling variation in 

 forelimb eruption. 



There is a difference of opinion as to the condition in normal 

 frog larva:-. Barfurth ("8") in Europe finds in the case of Rutui 

 fitsca that 80 per cent, put out the right forelimb first, the left 

 almost always following in from two to eight hours. On tin- 

 other hand, < '.ndeniatsch ('14), using Ratni teniporaria and A' 

 CMiilcntd, Mate- that he has always found the rc\er-e to be the 



: i.e., in about 80 per cent, the left forelimb erupt.- hi 

 I )icker.-<-n '20) observes that in normal bullfrog and green frog 

 metamoniho-i- tin- left forelimb is usually put out first. My o\\ n 

 observations upon the normal tadpoles of Rana sylvatica, Rami 

 ilanniln, Runa itfsbeiana, and Ilyla crucifcr lead me to agree 

 with < 'iiideniai-ch and I >ickerson that in a definite majority of 



' a the left limb is the first to erupt. 



M \IIRIAL, EXPERIMENTS AND < >n-i i<\ \TIONS. 



The material used includes about 800 tadpole- of Ilyla cr; 

 ]\>!>ni , /;;<;, R<:mi chinnita, and Rann sykntim, and a te\\ 



of Rann (diitahri^ii-nsis and Rana f>ij>i<ns. The bullfrog and 

 : d|>ole- were collected at Charlotte-\ ille, \ 'ir^inia. 



tin' oth-r- at \\ I- Hole, Massachusetts. I ntreated animal- 



\\en- ii-iiall\- ke|>t in aquaria containing pond water and weed. 

 Administration of endocrine extract was accomplished by placing 

 some of tlu- extract in the water with the animals. Thyroid and 

 thymus de-iiv.ited extracts were used. 



The accompanying table indicates the relali\e frequency of 

 left or riijit fon-limb eruption, as it occurs under normal condi- 

 tions, under thyroid treatment, and under thymu- treatment. 



1 Gu<lcrnat<rh makes thu -t.it-nn-nt in a footnote referring to his experinx nt 

 with thyroid-accelerated metamorphosis It is probable, therefore, that he in- 

 cluded his observations on thyroj.l-trcatcd animals with those on normal animals 

 in regard to forelimb eruption, not realizing that thyroid treatment affects forelimli 

 eruption. Thyroid administration, as shown by this paper, markedly favois tin 

 prior eruption of the left forelimb. His percentage, therefore. i< not correct, l>ut 

 is too high in favor of lefthandecli 



