THE EPIGENETICS OF THE EMBRYONIC AXIS 203 



the ventral side (measured in terms of nitrogen content of the whole cell). 

 There is, however, very little glycolysis in any part of the embryo when 

 oxygen is available. 



There seems good evidence, therefore, that the organiser region is 

 characterised by a particularly active breakdown of glycogen, although 

 it does not absorb more oxygen than other comparable parts of the 

 embryo. It is tempting to suppose that this carbohydrate metaboHsm may 

 be connected with the release of the evocator within the invaginating 

 mesoderm; but that conclusion is not the only one which might be put 

 forward. It is, perhaps, even more probable that the breakdov^Ti of glyco- 

 gen provides in the main the energy which must be utiHsed in the per- 

 formance of the movements of invagination (cf. Jaeger 1945). The direct 

 oxidation of glycogen is not essential for gastrulation, since many species, 

 particularly of toads, can gastrulate under anaerobic conditions, or in 

 concentrations of cyanide which inhibit 90 per cent of the normal 

 oxygen uptake; some other species (e.g. the frog) are more sensitive and 

 unable to gastrulate under such conditions. (Cleavage is always relatively 

 independent of oxygen in the amphibia). However, Brachet (cf. 1944) 

 found that iodo-acetate, which inliibits the breakdown of glycogen both 

 aerobically and anaerobically, brings gastrulation to a standstill without 

 impairing the inductive power of the organiser. Although the arrest of 

 movement does not occur tiU the yolk-plug stage, this evidence rather 

 supports the suggestion that the glycogen is being used to provide energy 

 for invagination rather than in direct connection with the evocator. 



Barth (see Barth and Barth 195 1) is studying the mechanism by which 

 the embryo utihses the energy derived from glycogen, and also that made 

 available when the yolk is digested at a later stage. He finds that high- 

 energy phosphate bonds, such as those in adenosine triphosphate, are 

 involved, and suggests that the chemical system has some similarity with 

 that characteristic of muscle (see also Dainty et al. 1944 and Fujii et al. 

 1951). 



At the beginning of gastrulation, certain other alterations take place in 

 the metaboHsm of the egg, and again certain of them appear to go fastest 

 in the blastospore region. A fact which emerges from cytological observa- 

 tion is that in the gastrula the nuclei become suddenly smaller and stain 

 more deeply with DNA dyes (such as the Feulgen reagent) while nucleoli 

 containing RNA make their appearance. These changes presumably 

 indicate an increased synthesis of nuclear RNA and possibly of DNA too 

 (Brachet 1952^). They occur more or less simultaneously throughout the 

 animal cells, but more slowly in the endoderm, where the nuclei re- 

 main large. Sirlin and Waddington (1954) found, in autoradiographs of 



