134 CM. JACKSON 



0.76 per cent at ten weeks. In terms of absolute weight, the 

 eyeballs have apparently increased from an average of 0.120 

 grams at three weeks to about 0.179 grams (no correction made 

 for the slight difference in body-weight) at ten weeks, an increase 

 of nearly 50 per cent! In the normal, full-fed rat at ten weeks 

 (average body-weight 112 grams) the eyeballs have reached a 

 weight of only about 0.201 grams (Jackson '13). At this rate, 

 the weight of the eyeballs at a normal body-weight of 75 grams 

 (the body-weight indicated in table 12 as 'normal at 10 weeks.' 

 to correspond to the body-weight of the animals held at constant 

 body-weight from the age of ten to thirty-five weeks) would be 

 only about 0.173 grams, or slightly less than that actually reached 

 in the series held at constant body-weight of 24 grams from three 

 weeks to ten weeks of age. The growth of the eyeballs in rats 

 held constant from the age of six to thirty-two weeks, and from 

 ten to thirty-five weeks, is equally striking. 



No data upon the growth of the eyeballs under these conditions 

 have been found in the literature. I have shown elsewhere 

 (Jackson '15 a, '15 c), however, that the eyeballs lose but very 

 little if any during inanition in the adult albino rat. 



In connection with the astonishing growth capacity of the 

 eyeballs in young animals at constant body-weight, the possi- 

 bility that the growth of the eyeballs is somewhat independent 

 of that in the body as a whole may be considered, which I have 

 already pointed out (Jackson '13, p. 24). When the large water- 

 content of the eyeballs is considered (85.6 per cent in the rat at 

 twenty days, according to Lowrey '13), it is, after all, not diffi- 

 cult to comprehend the possibility of its continued growth, largely 

 by water-absorption, when growth in the body as a whole is at a 

 standstill. 



THYROID GLAND 



In young albino rats held at constant body-weight from the age 

 of three weeks to six weeks, eight weeks and ten weeks, there is 

 usually a well-marked loss of weight in the thyroid gland (table 

 13). In the largest group, three to ten weeks, the thyroid has 

 apparently decreased on the average from about 0.033 per cent 



