EFFECT OF EXTIRPATION OF THYROID 595 



Average volume of thymus glands of: 



Controls 



Normal tadpoles, Specimens 5 to 10 1 .293 



Young frogs, Specimens 11 to 16 60S 



Young sexually mature frogs, Specimens 18 to 24 182 



Thyroidless tadpoles, Thyroidless 7 to 10 802 



Contrary to this, control 7, a 30 mm. tadpole has a pituitary 

 gland represented by 190 while control 8, a 31 mm. tadpole has 

 a pituitary gland represented by 127 (table 2). In this case 

 the smaller tadpole has the larger pituitary gland thus showing 

 individual variation. In these cases the thymus glands am 

 larger in the animal containing the larger pituitary gland. 



There are several reasons for this degree of variation. In 

 the first place tadpoles reared in a natural environment show 

 marked differences in rate of development even when derived 

 from the same mass of eggs. When placed under artificial 

 conditions in the laboratory these differences are accentuated. 

 This is especially true of the later stages. Such differences are 

 of influence in bringing about differences in the quantity of 

 food, oxygenation, etc. This was accentuated by the fact that 

 these tadpoles developed twisted tails as described in Dr. Allen's. 

 paper. 4 As regards the thymus gland it is well known that this 

 is an extremely variable structure in man. Further study may- 

 show the same thing to be true in the frog. The most extreme; 

 variation in these glands as seen in the controls is found at the 

 period immediately after metamorphosis. This is to be ex- 

 pected because of the sudden and profound changes that most 

 of the organs of the body have undergone. A slight variation 

 in the rate of development will bring about marked individual 

 differences in the volume of these glands. 



With such a small number of specimens available it is not 

 claimed that the results of this work are quantitatively accurate 

 within a wide range of probable error; but the results seem suf- 

 ficiently consistent and definite to justify the conclusions drawn. 



In some specimens one thymus gland is larger than the other 

 of the pair. A case of this kind is found in control 6 (table 



4 Allen, B. M. 1917 Jour. Exp. Zool., vol. 24. 



