SECT. 7] OF EMBRYONIC DEVELOPMENT 977 



says Stephenson "one may regard the evolution of the metazoal 

 organism as involving a process whereby the energy liberated in 

 chemical activity, which in a microbe runs to waste, is so organised 

 and disciplined that it is liberated when and where it can subserve 

 function, such as muscular work or maintenance of temperature; 

 apart from such organised expenditure the liberation of energy in the 

 higher animal is cut down to a minimum represented by its basal 

 metabolism or energy of maintenance." She left the question open 

 as to whether this latter quota might be, even in mammals, an ex- 

 penditure which the animal was unable to prevent or, conversely, of 

 some deeper significance. Her picture of the gradual increase of 

 organisation in evolution, is one of much interest and may apply also 

 to the ontogenetic passage from low to high efficiency seen in Fig. 260. 

 Possibly the chick embryo in its earliest stages may resemble the 

 micro-organisms in being unable to keep its enzymes apart from its 

 substrates, although it may be noted that its efficiency is not lower 

 than 40 per cent, at the worst^. 



A third way of considering the phenomenon of rising efficiency is 

 that of Terroine and his collaborators. In their studies of the germi- 

 nation of seeds (see Appendix iii) they found that — roughly speaking — 

 the A.E.E. was highest when the reserves were mainly in the form of 

 carbohydrate, mediocre when the reserves were in the form of fat, 

 and lowest when they were made up of protein. Now the seedling 

 itself, which corresponds to the developing embryo, may be considered 

 as made up almost entirely of cellulose, i.e. carbohydrate, and Terroine 

 and his colleagues therefore concluded that the A.E.E. varied with 

 the nature of the food-materials, i.e. was a measure of the degree of 

 chemical difference between the reserve materials and the finished 

 structure. The least wastage occurred when carbohydrate was used, 

 more when fat was used, and most when protein was used. In their 

 work on germination it was assumed (for the sake of argument) 

 that the composition of the seedling and the reserves was through- 

 out the same, and this was justifiable enough as the A.E.E., estimated 

 at various moments in germination, was always found to be the same. 

 But in the bird's egg, the composition of the embryo does not remain 

 the same; profound changes are taking place all the time in its 



1 Cf. the condition seen in the unfertilised echinoderm egg (p. 626) . But the embryo 

 in the early stages seems not only to combust an excess of nutritive material, but also to 

 fail to retain properly those building-stones which it does not combust, judging from the 

 high proportion of amino-acid nitrogen in the allantoic liquid at that time (see p. 1096). 

 Could this also be of ancestral significance? (see Table 163). 



