390 ON INCREASE IN SIZE [pt. iii 



altered this formula to 5-6. Again, Mall's rule states that the number 

 of days embryo age is equal to the square root of the foetal total 

 length in centimetres x 100. Balthazard & Dervieux have also 

 evolved formulae relating foetal age to the length of the limb-bones, 

 e.g.: 



L = femur length x 5-6 + 8 cm. 



L = humerus length x 6-5 + 8 cm. 



L = tibia length x 6-5 + 8 cm. 



The use of empirical formulae in the description of human foetal 

 growth has been carried to its greatest refinement in the work of 

 Scammon & Calkins, whose formula, 



n- 2-5/, L2 



holds with great exactitude from the third month onwards. Another 

 of their formulae, 



T = 2-134 X o-iZ X o-ooiiL^, 



holds with rather less exactitude from 2-5 foetal months onwards. 

 In both these cases, T is the menstrual age in lunar months, L the 

 total or crown-heel length of the dead body in centimetres. They also 

 found that 



W= (o-26L)3-i'>8 + 4-6, 



3 108 , 



or Z, = 3-846 VW - 4-6, 



where W is the weight of the dead body. From these equations, it 

 follows that 



3 108/ 15S4 / 



T= 2-134 + 0-3846 VW — 4-6 + 0-01627 VW — 4-6, 

 or T= 3-0 + ^•04.gVw — 0-012, 

 orW= 0-561 — 0-366 T X 0-061 T^. 



The formula of Donaldson, Dunn «& Watson, for the post-natal 

 growth of the white rat- up to 80 days, W = a + bT + cT^, and after 

 80 days, W = a log T — bT — c, was of the same type as the other 

 equations mentioned, but it had the additional refinement of in- 

 cluding constants, a, b and c, which were variable according to sex 

 and other factors. 



Murray, in his study of the chemistry of embryonic development, 



