ENDOCRINE GLANDS AND BILATERAL SYMMETRY: 



OBSERVATIONS UPON FORELIMB ERUPTION IN 



FROG LARV/E UNDER TREATMENT WITH 



THYROID AND THYMUS EXTRACTS. 



C. C. SPEIDEL. 



From the Laboratory of Histology and Embryology, 

 University of Virginia Medical School. 



INTRODUCTION. 



A conspicuous feature of thyroid-accelerated metamorphosis 

 in frog larvae is the eruption of the forelimbs. Before these 

 become visible externally they are located beneath the skin in 

 the gill chamber. For some years the writer has been aware of 

 the fact that the left forelimb is protruded before the right 

 forelimb, sometimes by as much as several days. This condition 

 was recorded in a previous paper (Jordan and Speidel, '23) and 

 has probably also been noted by other workers in the field of 

 amphibian metamorphosis. In half-grown bullfrog and green 

 frog tadpoles no exceptions to this were seen; i.e., no case ap- 

 peared of precedent right forelimb eruption in thyroid-treated 

 tadpoles. It was, therefore, of some interest to find in a jar of 

 seven thymus-treated tadpoles one animal in which the right 

 forelimb appeared two weeks before the left. Furthermore, in 

 two other animals in this lot the amount of skin degeneration 

 in the right forelimb region was definitely farther advanced than 

 that on the left side, a condition indicating probable prior right 

 forelimb eruption. Death ensued, however, before the appear- 

 ance of either forelimb. Two of the other four animals in this 

 jar put out the left forelimb first, and in one the amount of skin 

 degeneration on the left side plainly foreshadowed the prior 

 eruption of the limb of that side. The remaining animal put 

 out both limbs over night. In this jar of seven thymus-treated 

 animals, therefore, the ratio of "right-handed" to "left-handed" 

 animals is I : I. 1 



The question suggests itself as to whether the endocrine secre- 



1 The terms "right-handed" and "left-handed" are used to denote merely prior 

 right forelimb eruption or prior left forelimb eruption, respectively. 



336 



