SECT. 2] 



AND WEIGHT 



433 



■ whole embryo (Murray 



and Needham) 

 Bcalorlfic value (Murray) 

 Ddry solid (Murray) 

 Ocarbohydrate (Needham) 

 ® protein (Murray 



and Needham) 

 e fat (Murray) 



cent, per day. The lowest rate of growth ever reached by the rat 

 after birth is 3 per cent, per day. Given percentage rates of growth, 

 therefore, do not signify equivalent stages of development irrespective 

 of the species of animal. 



Calculation of the rates of growth for various processes and indivi- 

 dual components in the development of the embryo has also been 

 done by other investigators using the Minot method. In 1927 

 I calculated the percentage growth-rate for the total carbohydrate 

 content of the chick embryo; it fell from 56 to 22 per cent. 

 In Fig. 54 is shown the fall in 

 the Minot curves for the wet 

 weight of the whole embryo, 

 the calorific value, the dry 

 weight of the whole embryo, 

 the sugar, protein, and fat 

 content of the embryo. All of ^° 

 them fall, but we have here an 

 instance of the limited but real 

 value of the Minot curves, 

 which, although no absolute 

 conclusion can be drawn from 

 them, do show that the tissue 

 constituents and the dry weight 



have a different behaviour from the wet weight. It can easily be 

 seen that they form a plateau between the loth and the 15th day, 

 during which they grow at a constant rate while the wet weight is 

 falling all the time. This plateau also appears on the curves for carbon 

 dioxide output calculated in the same manner as percentage growth- 

 rates from the figures of Bohr & Hasselbalch; Atwood & Weakley; 

 and Murray in 1927 by Brody. The plateau must be due to the fact 

 that the growth of dry substance is specially rapid during the middle 

 phase of development; it is then that the embryo makes the most 

 rapid strides from wetness to dryness. It is interesting to see that 

 the growth-rate of carbohydrate is never as high as some of the 

 others, and never drops so low. It is significant, moreover, that 

 on the 1 9th day the Minot growth-rate of the protein has dropped 

 below that of the whole body, while the growth-rate of fat remains 

 well above it. This is an illustration of the "relative" use of Minot's 

 method. 



Fig- 54- 



88 



