KNLHX RINK (, LANDS AND BILATKKAI. SYMMETRY. 343 



much importance. In about 30 per cent, of cases the right fore- 

 limb succeeds in overcoming the handicap of having no spiracle 

 to come through, and breaks through by main strength before 

 the left. In the other 70 per cent, the left limb aided by the 

 sinistral spiracle comes through first. A majority in favor of 

 lefthandedness i- about what should be expected. 



Mention -hould be made again of Barfurth's results. In 

 normal Rana fn.si tadpoles he finds that 80 per cent, put out the 

 right forelimb lir-t. Presumably these art- like all other fro- 

 tadpole- in having -mistral spiracles although Kirfurth does not 

 reier to tin- le.ttnre. He believes the prevailing righthandedness 

 ot tin- -pe< ii - is accounted for by two factors: (l) earlier and 

 greater degeneration of the skin in the right forelimli region, and 

 (2) greater -i/e and strength of the right limb. As the-e n -alt- 

 are din-< ilv opposed to the observations of < indernatsch. I >icker- 

 son and m\-clt on five species of frog ta<lpoles, it can onl\ be 

 -npp-ed ih.it there is a species difference, and that the la* tor- 

 nientioned by Harfurth are strong enough to bring about a 

 m.roiiiy of righthanders in this particular -pecie-. It would be 

 of intcre-t to see whether in this s|>ecie- also the uniform let! 

 handed '"iidition could be produced by thyroid treatment. 



The explanation of the results after thymus treatment i- 

 -oine\\ h.u more difficult and uncertain. It is probable that a 

 -hilt louard righthandedness is here indicated. The ratio max 

 be I : I, although on account of the -mall number of aninial- 

 ol>-ei \ed iii the three-limb stage, this is by no means a certainty. 

 At .m\ rale, tin- It ening of the lefthanded majority means thai 

 the a-\ innieii ii al position of the spiracle befmiie- of much k 

 importance as a factor in determining the in -i ton-limb to appear. 



iV-nvaud thxiiuis extract is a food rich in nutritive valin, 

 and therefore favorable to growth. Giulernat-. h ('14) noted i^ 

 growth-promoting elUrt upon tadpoles and a-Mil.i'd it to the 

 endoiriiie -i-.retion of the thymus. rhlenhulh ('17.1 though he 

 i-ombat- .ndernatM -\\\ idea as to the grow th elieet being due to 

 an endocrine Accretion contained in the thymu.*. extratt, states 

 that it i> a \ery rich and nutritious food and therefore <|uite 

 taxorable to gro\\th. The writer has al-o ol>-er\ed that it i- 

 particularlx- ta\oral)le to limb growth. In one batch of partly- 

 n th\ inns-treated v:reen frog tadpoles the small hind limb- 



