386 



ON INCREASE IN SIZE 



[PT. Ill 



the result that the curve slackens off and enters a slowly-rising phase 

 again. It is possible, of course, to calculate the average daily incre- 

 ment for the whole developmental period (see Table 52), and the 

 figures so obtained have been made the basis of a comparison of 

 animals by Friedenthal. The fourth curve, that of the percentage 

 growth-rate, Minot's curve, as it may be called, begins at a high 

 level and continually descends, although in this instance there is a 

 slight kink on it midway through development, which may for the 

 moment be disregarded. All Minot curves begin at a high level and 

 descend as development proceeds. Now, in many cases, it may 

 happen that not only the increments but also the whole growth- 

 process itself slackens off towards the end of the period taken, in 

 which circumstance the curve relating weight of animal at any 

 given time to age will also have an S-shaped form. It has not 

 escaped the perspicuity of those who have considered these pheno- 

 mena that this S-shaped curve has a resemblance to the S-shaped curve 

 of an autocatalysed monomolecular reaction, and this likeness will 

 shortly be taken up at length. 



Table 52. Average daily increments. 

 Friedenthal's figures. 



A more complicated example of the various types of growth-curves 

 is afforded by Fig. 35 taken from Brody. It shows the growth 

 throughout the life-span of the albino rat. The curve passing through 

 the circles shows the course of growth ; it is, in fact, the weight of 

 the whole animal at any given moment plotted against the age 

 at that moment. The strongly indented curve, passing through the 

 crosses, is the line showing increment in unit time. In the data of 

 Murray for the embryo chick the absolute growth-curve rises steadily. 



