SECT. 2] 



AND WEIGHT 



399 



of the epithelium of the foetal lung is, however, of greater 



interest i 



W = At^w + Bfiw + Ctw + Dw, 



where w is the total weight of the lung at the time in question. But 

 here the number of arbitrary constants is so large that we reach the 

 point where the question naturally arises whether an empirical 



/q^ 0,1 p 



7 8 3 10 i1 12 13 1'^■ 15 16 17 18 19 20 21 

 Fig. 42. 



formula is worth looking for at all. The more complicated it is, 

 the less valuable it is, in view of the fact that, in any case, it is 

 not intended to give us an idea of the basic factors underlying the 

 process. 



2-5. Percentage Growth-rate and the Mitotic Index 



We have so far been examining the results of those investigators 

 who have taken the curves obtained by simply plotting the weight 

 of the embryo each day during development against the time, and 

 who have endeavoured to find a correct mathematical expression 

 for them without a preconceived theory. We have now to consider 



