INDIVIDUATION — FORMATION OF PATTERN AND SHAPE 455 



Selman (1955) has made similar measurements of the force of neuralisa- 

 tion ; his method consisted in placing two minute steel dumbbells against 

 the two neural ridges, and holding them apart by placing the whole 

 preparation within a coil carrying an alternating current; he found a 

 value of 40 X 10 ^ dynes. Using an estimate of the distance over which the 

 tissues move, one can roughly estimate the total amount of work accom- 

 plished: during gastrulation it is about 3 x lO"^ cal. per hour. Even if 

 one supposed that the energy-producing mechanism was working at an 

 efficiency of only 10 per cent, the amount of oxygen consumed during 

 the period could produce about a milHon times as much energy as this, 

 so that these figures suggest that morphogenetic work demands only a 

 very minute fraction of the energy available. It should be emphasised, 

 however, that the measurements of the forces are of a very prehminary 

 character; they may be too small, because the cells have been damaged 

 during the experiment, or alternatively they may be larger than the forces 

 actually exerted, since the measurement is of the maximum the tissues 

 can put out rather than what they normally do. 



A measurement of the force of gastrulation has been made by another 

 method in echinoderms by Moore (1941, 1945). If an echinoderm egg is 

 cultured from fertihsation onwards in sucrose solution, some of this be- 

 comes enclosed in the blastocoel, but when this cavity is fully formed the 

 solution cannot escape since the walls of the blastula are impermeable to 

 the sugar, which therefore exerts an osmotic pressure, directed outwards, 

 if the blastula is transferred to sea water. By finding what concentration 

 of sucrose just prevents the in-pushing of the endoderm, Moore decided 

 that the force of gastrulation in this form is about 5 gm. per mm.^. This 

 is about 10* as great as that found for the very different amphibian gastru- 

 lation process. Even so, as Moore points out, the work done in echinoderm 

 gastrulation would only demand about one thousandth of the oxygen 

 which is actually consumed. 



6. Individuation of the central nervous system in Amphibia 



The development of the nervous system from the gastrula to the neurula 

 stage in Amphibia provides probably the best example from which one 

 can get an idea of what is actually involved in the individuation of most 

 embryonic organs. Not only has the process been very thoroughly studied 

 in a long series of experiments by many authors, but it involves simultan- 

 eously both the aspects of individuation — morphogenesis and pattern 

 formation — which we have just considered separately. In reahty they 

 must usually occur in combination with one another, and the develop- 

 ment of the nervous system therefore gives a more generally valid picture 



