SECT. 2] AND WEIGHT 403 



and end of the week are nearly the same as compared to the gain in 

 weight. But to express the gain in weight during a week as a per- 

 centage of the weight of the body at the beginning of the week for 

 a 7-day old chick embryo would be quite fallacious. It would cor- 

 respond to expressing the gain in population in the U.S.A. from 1666 

 to 1927 as a percentage of the size of the population in 1666. The 

 growth of the population of the U.S.A. in 1927 is proportional to the 

 population in 1927 and not to the population in 1666. Similarly the 

 number of cells produced in a 7-day old chick embryo should be 

 functionally related to the number of reproducing cells (i.e. the 

 weight) of the chick at 7 days of age and not to the number of cells 

 at I day of age. In brief, growth is a continuous process and the 

 rate of growth at every instant should be functionally related to 

 the number of reproducing units at the given instant and not to 

 the number of reproducing units which existed in some relatively 

 remote past". In other words, Brody would prefer to ask not how 

 much 100 gm. of embryo add on to themselves during the im- 

 mediately succeeding period, but rather how many grams of those 

 1 00 gm. had been added on during a short preceding period. Murray's 

 modification of Minot's method, in which the mid-increments in- 

 stead of the daily increments are used as the basis for calculation, 

 goes some way to meet Brody's criticism, for it enquires how much 

 100 gm. of embryo add on to themselves during half the preceding 

 and half the following period, thus speaking in terms of a more 

 instantaneous measure. Brody himself has made use of a similar 

 amelioration. However, Brody's real point is that the fault lies in 

 choosing an arbitrary length of time interval for all stages of develop- 

 ment, in spite of the fact that they cannot possibly be equivalent for 

 the embryo. 



Brody might say that the embryo cannot be regarded as having 

 been given an equal chance to accomplish its growth in each of the 

 daily periods throughout its development. On the other hand, it 

 might be argued that, though this is doubtless true as regards growth 

 in weight, it is not true with respect to the activities of the embryo 

 as a whole, which include many other processes, such as chemical 

 differentiation. Taking the embryo as a whole in all its activities, 

 the arbitrarily chosen and invariant period might be regarded as an 

 adequate one. As we shall see later, this is essentially the same 

 argument as that used by Murray against Robertson. 



26-2 



