POSTNATAL GROWTH IN THE ALBINO RAT 33 



When grouped according to age periods (table 10, fig. 3 b) 

 my own data show an increase in the relative size of the (male) 

 spleen frdm 0.22 per cent of the body weight at birth to a maxi- 

 mum of 0.41 per cent at seven days. For succeeding ages, the 

 percentage weight agrees fairly with that derived from the for- 

 mula, excepting the unusually high figure (0.396 per cent) at 

 one year. There is no distinct difference between the sexes, or 

 change in relative size according to age (after the first week). 



It may be noted that my data for the spleen tend to run some- 

 what higher than the curve derived from Hatai's formula. This 

 may be because Hatai's formula is based upon data from which 

 the 'enlarged' spleens had been excluded. His figures for all 

 spleens average very much higher than mine. 



The spleen is known to be an unusually variable organ and 

 this is certainly true for the rat. As shown in table 10, the 

 coefficient of variation in absolute weight is from 25 to 51 (ex- 

 cepting at five months, 18.9) The coefficient of variation in per- 

 centage weight is usually but slightly less than that in absolute 

 weight. The coefficient of correlation between spleen and body 

 weight is accordingly somewhat low, varying from 0.406 to 0.542 

 with an exceptionally high correlation (partly 'spurious') of 0.967 

 at twenty days. 



10. Stomach and intestines 



From the curve of relative (percentage) weight constructed 

 from Hatai's ('13) formula (fig. 5), it appears that the empty 

 alimentary canal increases from about 3.0 per cent of the body 

 at 10 grams, to 7.5 per cent at 20 grams, and a maximum of 8.0 

 per cent at about 35 grams, after which it decreases to 6.3 per 

 cent at 100 grams, 5.0 per cent at 200 grams, and 4.5 per cent 

 at 300 grams. 



Observations were made upon the empty intestinal canal in 

 only a part of my own series. These observations, grouped 

 according to ages (table 11, fig. 5) indicate that the relative weight 

 of the empty tract increases from an average of about 2.4 per 

 cent of the body weight in the newborn to a maximum of about 

 8 per cent at six weeks, decreasing to 5 per cent at one year. 



THE AMERICAN JOURNAL OF ANATOMY, VOL. 15, NO 1 



