POSTNATAL GROWTH IN THE ALBINO RAT 49 



the females approaches or slightly exceeds that of the males. 

 After six weeks, growth is more vigorous in the males and the 

 females lag behind. 



2. Variability in body weight is lowest at birth (the coefficient 

 being about 12) and is not much higher at seven days (16). It 

 appears highest at three weeks (28), and at later periods varies 

 from 19 to 21. The average coefficient, taking all ages together, 

 is 19. There is in the rat no evident correlation between varia- 

 bility and rapidity of growth. The coefficient of variation is 

 practically the same for the gross as for the net body weight. 

 The coefficient usually appears higher in the male, but the differ- 

 ence is slight and of doubtful significance. 



3. Fraternal variability (within the litter) in body weight is 

 very low, usually less than half that of the total population of 

 the same age. 



4. As the growth rate of the whole body is a resultant of the 

 varying growth rates of the component systems (musculature, 

 skeleton, viscera, and so forth) so the growth rate of the vis- 

 ceral group is a resultant of the different growth rates of the 

 individual organs. While the visceral group as a whole reaches 

 its maximum relative (percentage) weight at about the age of 

 three weeks, the thyroid gland apparently is relatively largest at 

 birth, the brain, spinal cord, eyeballs, lungs and spleen about the 

 second week, the thjonus, heart, suprarenals (?) and kidneys at 

 three weeks, intestinal canal and liver at six weeks, and the 

 gonads at ten weeks. Differences according to sex, aside from 

 the gonads, are most marked in the eyeballs (?) and suprarenal 

 glands. 



5. Similarly the variability in the weight of the body as a whole 

 depends upon the variability of its component parts. In this 

 respect the individual organs may be classified in three groups: 

 (1) the head and head organs (brain, eyeballs) form a group of 

 comparatively slight variability (average coefficient 10 to 12) ; (2) 

 the heart, lungs, liver, suprarenals and kidneys form a group of 

 moderate variability (average coefficient 21 to 26) ; (3) the thy- 

 mus, spleen, gonads and intestinal canal (with contents) form 

 an extremely variable group (average coefficient above ,29). 



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



