INDIVIDUATION — FORMATION OF PATTERN AND SHAPE 453 



per unit mass than the normals. Tyler argues that this is a reflection of 

 the varying amounts of morphogenetic work per unit mass which the 

 different types have to perform. This may indeed well be so; but, as 

 Needham (1942) points out, the slow-developing half embryos have not 

 only got to do twice as much morphogenetic work per unit mass as 

 the normals, but also have to maintain themselves for longer before 

 reaching any particular stage; and it remains obscure how much of the 

 extra energy goes to one purpose or to the other. 



If considerable quantities of energy were utiHsed for the performance 

 of physical work, then there should be a measurable discrepancy between 

 the decrease in the calorific value of an embryo as its yolk is consumed 

 and the amount of heat which it gives out. The most thorough study of 

 this question is that of Smith (1946), and no such difference was found. 

 Various other authors have attempted to estimate the maximum fraction 

 of the UtiHsed energy which can possibly be supposed to be devoted to 

 such work. Tyler gives a figure of 30 per cent for echinoderm eggs. 

 Tuft (1953) has reviewed the measurements on the rate of oxygen con- 

 sumption which have been made on the developing eggs of a number of 

 different species (insects, fish, Amphibia). He shows that the curves are 

 often by no means simple, but may have a succession of phases of increas- 

 ing, stationary or even decreasing rates. In the bug Rhodnius, the rate of 

 oxygen consumption falls during a certain period. Tuft makes a calcula- 

 tion, based on the supposition that the minimum of the rate indicates the 

 maximum consumption which can be considered necessary for mainten- 

 ance, and concludes that it is conceivable that as much as 15 per cent 

 could be devoted to something else, such as morphogenetic work. The 

 weak point in such an argument is of course the assumption that the 

 maintenance requirements remain constant (Fig. 20.22). 



Little is as yet known about the biochemical systems by which energy 

 is dehvered to the morphogenetic mechanism; probably they involve 

 high-energy phosphate compounds (Barth and Barth 1951). The ease 

 with which a process such as gastrulation is brought to a standstill (e.g. 

 by thermal shocks, a wide range of chemical inhibitors, etc.) suggests 

 that the process is a very sensitive one. The ultimate source of the energy 

 for amphibian gastrulation is presumably caroohydrate, since, as we have 

 seen (p. 203), the consumption of glycogen increases greatly in the blasto- 

 pore region just when movement begins. It may well be, however, that 

 other morphogenetic processes obtain their energy from other sources. 

 Needham (193 1) claimed that it is a general rule that during embryonic 

 life, the predominant source of energy is, in the earHest stages, carbohy- 

 drate, then protein and fmally fat. More recent investigations (e.g. 



