POSTNATAL GROWTH IN THE ALBINO RAT 29 



at birth and seven days is much lower (0.583 and 0.504). How- 

 ever, the high figures for the older rats are partly due to a 

 'spurious correlation/ as will be explained later. 



7. Lungs 



According to calculations made from Hatai's ('13) formula, the 

 lungs, as shown by the curve in figure 2 a, decrease in relative 

 (percentage) weight from a maximum of 1.60 per cent of the 

 entire body weight at 5 grams to 1.32 per cent at 10 grams, 

 1.06 per cent at 20 grams, 0.80 per cent at 50 grams, 0.64 per 

 cent at 120 grams, 0.59 per cent at 200 grams, 0.55 per cent at 

 300 grams, and 0.54 per cent at 400 grams. 



The relative weight of the lungs in the male according to my 

 data increases from about 1.59 per cent of the body weight at 

 birth to 1.93 per cent at seven days. From this maximum it 

 decreases to 0.68 per cent at six weeks, and 0.57 per cent at ten 

 weeks. The apparent increase at five months and one year is 

 probably due to the inclusion of unrecognized pathological cases. 

 Except at twenty days, the relative weight of the lungs appears 

 slightly higher in the female than in the male. This is chiefly 

 because the body weight is usually greater in the male. Any 

 sexual difference aside from this is very doubtful. It will be 

 observed that the curve derived from Hatai's formula does not 

 show the preliminary increase in percentage weight which is 

 indicated by my data, and which is found so characteristic of 

 the viscera in general. 



The variability of the lungs in absolute weight at birth (23.3) 

 is much greater than that of the whole body, but is later approxi- 

 mately equal to it in most cases (16.6 to 23.9). The variation 

 in percentage weight is usually much less than in absolute weight ; 

 and the correlation with the body weight is correspondingly high 

 (73.6 to 94.3 except at ten weeks, 62.3). The high figures are 

 partly due to a 'spurious correlation.' 



The weights of the right and the left lungs were also taken 

 separately, although not recorded in the table. The right lung 

 is always much larger than the left, the ratio being approxi- 



