CHAPTER XIII 



GROWTH 



I. Overall growth 



In everyday usage the word 'growth' is used to mean any type of in- 

 crease in size. This is obviously one of the important phenomena in em- 

 bryonic development and requires discussion. There have been two ways 

 of approaching the problem; one is content to accept the everyday 

 meaning of the word and to study the increases which take place in whole 

 embryos or in their parts; the other has attempted to start from some 

 more precisely defmed process of growth and to set up general norms from 

 which the facts as they appear in the development of particular animals 

 can be deduced as special consequences. This second attempt has not as 

 yet proved very successful but it will be easier to exhibit the complexity 

 of the whole situation if we start by discussing it (General Reviews : 

 Needham 193 1, Medawar 1945). 



If we wish to consider a precisely defmed process of growth we shall 

 have to fmd some way of limiting the concept so that it is confmed to the 

 increase in size of something which retains a certain similarity to itself. 

 Size may increase merely by the imbibition of water or by the laying down 

 of relatively inert material such as shell, bone, cartilage, etc., and such 

 processes obviously differ in kind from the increase in amount of the 

 living material itself. Various definitions have been offered with the 

 purpose of excluding them from the concept of growth as that is required 

 for a precise theory. Gray (193 1) speaks of growth as 'essentially concerned 

 with the formation of new living material'. Medawar (1941) states that 

 'what results from biological growth is itself typically capable of grow- 

 ing'. Weiss (1949) gives a more formal dcfmition; growth is 'the in- 

 crease in that part of the molecular population of an organic system which 

 is synthesised within that system', and he further amplifies this, pointing 

 out that it means 'the multiplication of that part of the molecular popula- 

 tion capable of further continued reproduction'. This puts its fmger on 

 the important point ; if we are trying to formulate a precise concept of 

 growth we must confme it to the increase in the amount of the system 

 which is capable of growing. The main general problem which then 

 requires study is this — at what rate does this increase take place and how 

 does the rate change as time passes ? 



The simplest possible situation would be one in which the rate of multi- 

 plication per unit mass remained constant. We could formulate this 



T 279 



