GROWTH 285 



progressively distending the eyeball. In addition, blood vessels and other 

 mesenchyme penetrate into the eye from the surroundings. 



'This diversity and complexity of the component processes contributing 

 to eye size makes the search for a single "growth-controlling" principle 

 appear utterly unrealistic . . .' 



Further, it must be remembered that the growth of an organism or 

 organ may be due to a multiplication of cells, which remain about the 

 same size, or to an enlargement of cells which do not increase in number. 

 The fmal number of muscle or nerve cells in a vertebrate, for instance, is 

 probably attained fairly early in embryonic life, the growth of the organs 

 thereafter occurring mainly, if not entirely, by increase in cell size. It is 

 not entirely clear whether the two types of growth depend on quite 

 different underlying synthetic processes, but there is obviously some con- 

 siderable difference between them, so that there is no reason to expect 

 that they would follow identical growth laws. 



Finally, it is possible for organs, or even entire organisms of relatively 

 simple structure, to 'de-grow' or become smaller. Flatworms or coelen- 

 terates may respond in this way to deprivation of food supplies. In higher 

 animals, the regression of a tadpole's tail at the time of metamorphosis 

 is a striking example. 



It is hardly to be expected that any very simple formula can fit all such 

 cases. Attempts have been made to elaborate more complex ones. Perhaps 

 the most valiant is that of Wetzel (1937) who set up an equation of extreme 

 complexity containing over a dozen different constants, each of which 

 was designed to deal with one or other of the factors which he supposed 

 to be involved in the growth of a heterogeneous collection of tissues, such 

 as an embryo. The formula was so complex, and therefore so flexible, 

 that it could have been made to fit almost any set of data. Its justification 

 would in fact have to be sought not in the accuracy with which it could be 

 fitted to observations of growth, but in the experimental justification of 

 the various parts of the formula which were concerned with the postulated 

 underlying processes. We still know far too little about the unit processes 

 which go to build up the overall growth rate of an embryo for such an 

 experimental justification to be provided. 



Most authors recently have been content to accept the situation that 

 the growth of a complex organism cannot either be formulated adequately 

 in terms of any simple, global hypothesis, such as those listed above, nor 

 can it as yet be adequately analysed into a series of part processes; whence 

 it follows that we have to use the various growth formulae merely as 

 convenient means of summarising the empirical observations without 

 attempting to attribute any profound meaning to them. For further 



