POSTNATAL GROWTH IN THE ALBINO RAT 25 



Furthermore, the idea of an independent growth of the eye- 

 balls is supported by the variation in the percentage weight rela- 

 tions (table 4). While in the newborn the coefficient of variation 

 in the percentage weight is less than that for the absolute weight 

 (the usual relation in other organs at all ages), it is a remarkable 

 fact that after seven days the variation in the eyeballs is decidedly 

 greater for the percentage weight. Finally, the coefficient of cor- 

 relation between body weight and weight of the eyeballs, as 

 might be expected from the foregoing, is comparatively low and 

 very irregular (0.21 to 0.67). 



4. Thyroid gland 



When the calculations are made according to Hatai's formula, 

 the relative weight of the thyroid (fig. 3 a) is greatest in the 

 earliest stages, forming about 0.029 per cent of the body weight 

 at 5 grams. The relative weight decreases slowly to 0.028 per 

 cent of the body at 10 grams, 0.026 per cent at 20 grams, 0.022 

 per cent at 50 grams, 0.019 per cent at 100 grams, 0.016 per cent 

 at 200 grams, and 0.015 per cent at 300 grams. 



On account of certain difficulties, the number of observations 

 upon the thyroid gland in my own series is somewhat limited. 

 The gland is small and difficult to separate accurately from the 

 adjacent muscles, especially in the younger stages. It was also 

 sometimes injured in the process of decapitation. The obser- 

 vations recorded, arranged according to age (table 5, fig. 3 a), 

 indicate that the thyroid is relatively largest in the newborn, 

 decreasing from about 0.04 per cent of the body weight to 0.018 

 per cent at one year. In comparison with Hatai's theoretical 

 curve of growth, there is in my data a lagging behind during the 

 first week. The thyroid apparently increases but slightly during 

 this period while the body weight doubles. This causes a con- 

 siderable drop in the percentage weight of the thyroid at seven 

 days. No difference according to sex is apparent. On account 

 of the limited data, the coefficients of variation and correlation 

 were not calculated. 



Watson ('10) finds the thyroid (and parathyroid) glands usu- 

 ally enlarged in rats fed upon meat, and especially oatmeal, diet. 



