EMBRYONIC GROWTH 



143 



It is seen that there is in actual fact (q values) a growth- 

 gradient in the 3rd digit dropping distally (with possible slight 

 rise quite terminally again) ; while if we do not take account 

 of the difference in time of origin, the gradient (k values) is 

 quite obscured and the terminal digit comes to have the 

 largest ' growth-coefhcient '. 



The change in relative size of various organs of the chick 

 during embryonic life is shown in Fig. 72, and the actual 

 growth of some others has been plotted from Schmalhausen's 



7 8 3 10 II 12 13 14 15 16 17 18 19 20 21 days 



Fig. 72. — Changes in relative size in various organs of the chick during 



embryonic life. 



B, brain ; H, heart ; F, fore-limb ; M, metanephros. 



Note the very different shapes of the curves. This depends (a) on the time of origin of the organ, 

 (fc) on its relative growth-rate. 



data in Fig. 73. The value of Schmalhausen's method is 

 clearly evident. It means that we cannot discover the true 

 growth-coefficient of an organ during its early stages without 

 precise information as to the time-relations of development. 



Somewhat unfortunately from our point of view, Schmal- 

 hausen prefers in general not to work with growth-quotients, 

 which are our growth-coefficients corrected for difference in 

 time of origin, but with growth-constants. These are obtained 

 by multiplying the growth-rate C v of an organ (see equation 1) 

 for a given period by the mean age of the organ during that 



