ENDOCRINE GLANDS AND BILATERAL SYMMETRY. 339 



the time of forelimb eruption, one righthanded animal occurred 

 during the first 24 hours; after that all were lefthanded. For 

 this reason, in Table I. all animals putting out forelimbs during 

 tin- first two day- are omitted from the reckoning, since the 

 original normal bias may not have been sufficiently influenced by 

 the endocrine extract. 



How long it take- for the thymus extract to affect .in original 

 bias is not known. Since it is probably not >o powerful as the 

 thyroid it m.iy be that more than the first two da\-' re-nlts 

 should be discarded. If the first two days arc di-cardcd the 

 ratio is 17 : 17 as given in the table. If the result- of the first 

 three to seven days are discarded the ratio shift- pro-re i\ely 

 to favor righthandedness. Only a much larger number of ani- 

 mal- under ol,M-f\ation would give a trustworthy ratio. The 

 figure- v,i\cn, however, are in all probability enough to -ho\\ 

 that the normal ratio has been affected and shifted in the d 

 lion of righthandedness. 



INTERPRETATION AND I)i- i --ION. 



The-M- ob-er\ ations leave no doubt that tin- bilateral symmetry 

 ol ilie de\-elopiiig frog tadpole, as indicated l-\ the relative time 

 of forelimb eruption, is influenced by thyroid extract; possibly 

 also by thymus extract. The action of tin- thyroid extract will 

 Mr-t be di-cu ed. The blood carries the active thyroid principle 

 to .ill part- of the bi.ih. It is inconceix able that the thyroid 

 autacoid -hoiild have one effect on the ti--m- on the left side 

 and another elteet on exactly similar tissue- on the right side. 



riierelore. the elteet of the thyroid in (hanging the normal 

 -\minetrical de\elopment must be due to some original funda- 

 mental asymmetry of the body pattern. 



The lolloping findings are pertinent : The tadpole i- coii-pieu- 

 ou-ly a-\ inmetric'al in respect to its respiratory ,i|ijiarat u-. A 

 spiraele. of outlet fmm the gill chamber, i- pre-ent on the lelt 

 side only ( Fig. I). This outlet drain- both left and right gill 

 chambers, these being connected by a canal across the mid-line. 



The torelimb is pre.-ent in the gill chamber, it- degree of develop- 

 ment depending upon the general developmental state of the 

 tadpole. Both in normal and thyroid-induced metamorphosis 

 23 



