30 E. R. HOSKINS AND M. M. HOSKINS 



metamorphosis) are more nearly spherical, and still others more 

 closely resemble parallelopipeds. The volumes of bodies of these 

 shapes vary greatly when considered in relation to their three 

 principal dimensions, and hence they are not comparable. 



Figures 4G and 47 are representative of the thymus as it exists 

 in operated and control larvae of the same size at a time just 

 preceding metamorphosis of the latter. Any difference between 

 the total volume of the two thymi of the thyroidless larva (fig. 

 46) and that of the control (fig. 47) is no greater than the normal 

 variability of the thymus within either group. During meta- 

 morphosis the thymus changes in shape and decreases in size 

 (fig. 48). Microscopic examinations showed no difference in 

 compactness or size of the cells of the thymus after metamorpho- 

 sis, and hence this decrease in size of the gland is due to an actual 

 decrease in the thjonic tissue. The change in shape of the 

 thymus at this time is due to change in the pressure from the 

 surrounding structures. There is a decrease in the volume of 

 the thymus during metamorphosis, the young frog has rela- 

 tively a larger thymus than the control larva, since the 

 volume of the larva decreases more at this time than does that 

 of the gland. This fact is overlooked by Rogers, who shows 

 (p. 595) that after he had 'corrected' the combined 'volumes' 

 of the thymi these glands were relatively about twice as large 

 in his control tadpoles as in the young frogs. 



Rogers ('18) compares the thymus of various groups of animals 

 with a single small thymus taken from a single animal (no. 4, 

 p. 594) and draws general conclusions from this (pp. 596 to 598). 

 He overlooked the fact that this particular gland is abnormally 

 small, and the other one of the pair which was not measured 

 might well have been quite large, as may be seen to be the case 

 in several of his specimens (p. 594) and as is very evident in our 

 own material. Had Rogers lost or destroyed the right thymus 

 of animal no. 4 instead of the left thymus as happened, or had 

 he killed his animal no. 8 instead of his no. 4 at his time, his 

 conclusions might have been greatly different from what they 

 were, in regard to the thymus. As a matter of fact, the average 

 size (volume or weight) of young frogs is from three to five times 



