l-.NDOCRINE GLANDS AND BILA11.KM. SYMMETRY. 345 



in any vertebrate animal in process of development : A change in 

 the normal balance of thyroid secretion may lead to a change 

 in the symmetrical development. Stockard ('23) has emphasized 

 the general importance of thyroid secretion in the development 

 of man and mammals and its part in determining the production 

 of definite types. These results on forelimb eruption in tadpoles 

 indicate th.it thyroid >ecretion may be of some importance also 

 in influencing symmetrical development. A vertebrate animal, 

 though de-ignau-d .is bilaterally symmetrical, is, of cour-e. a-yni- 

 mrtrical in m.m\ re-pects, e.g., the visceral pattern, much ol the 

 vascular -y-tem. etc. Given an original asymmetrical condition 

 it i- po^-ible th.it the thyroid may exert its effect upon the- t\\o 

 sides of tin- body in a differential way. 



SUMMARY. 



The bilaterally symmetrical development of the frog lar\a 

 i- altei ted in a definite way by experimental hyperthyroidism. 

 Normally in tadpole metamorphosis the left forelhnb erupt- 

 tir-t in about 70 per cent, ol cases. With thyroid-accelerated 

 metamorphosis of half-grown tad|>oles practically IOO per cent. 

 will put out the left forelimb first. Of 133 thyroid-treated ani- 

 mals of this kind every one protruded the left forelimb first. In 

 the case of full-grown tadjxiles already near the time of forelimb 

 eruption, th\roid treatment may be followed during the first 

 t\\o da\- l'\ -onie prior right forelimb eruptions ; thereafter prior 

 left forelimb eruption obtains. In other word-, an original bias 

 (.1 an animal toward prior right forelimb eruption may not be 

 changed by thyroid administration within t \\odays. 



Tin- eiie. i ..t the thyroid on symmetrical development is 

 explicable in term- of the original asymmetrical pattern of the 

 iv-pirator\ apparatus (/'. e., sinistrally located spiracle) coupled 

 with the close anatomical relation of the forelimb to this appa- 

 ratus. 



Th\ inn- treatment brings about a larger percentage of prior 

 right forelimb eruptions, thus reducing sdmcwhat the normal 

 majority in ta\or of prior left forelimb eruption. 



