THE EPIGENETICS OF THE EMBRYONIC AXIS 205 



oxygen consumption have so far failed to reveal any differences at the 

 various levels along the streak (Phillips 1942), but Jacobson (1938) has 

 shown that there is a rapid disappearance of glycogen during the invagina- 

 tion of the mesoderm, just as there is in Amphibia, Brachet (1944) finds 

 that there is also a concentration of basophilic substances and of -SH 

 containing proteins in the invaginating region, again as in Amphibia; in 

 the chick, in which invagination occurs earlier at the anterior end of the 

 streak, these substances are distributed in a gradient, decreasing towards 

 the posterior. A similar gradient has been found for an indophenol oxidase 

 (probably cytochrome oxidase) by Moog (1943). Rulon (1935) stained 

 blastoderms in oxidised Janus Green, and showed that, under conditions 

 of low oxygen tension, the dye was reduced fastest in the region of the 

 streak; again there was a gradient decreasing from anterior to posterior. 



Spratt (1952^7, h) has recently studied such reducing systems in more 

 detail, investigating the effects of aerobic as well as anaerobic conditions 

 and varying the nature of the carbohydrate substrates available to the 

 blastoderm. He made the very interesting observation that the node 

 region shows a high rate of activity under a wider range of conditions 

 than does the forebrain; Spratt suggests that we have here a chemical 

 manifestation of the fact that the node is still undetermined and labile, 

 whereas the forebrain has already entered on one particular and limited 

 course of differentiation. 



Feldman and Waddington (1955) have studied the incorporation into 

 early chick embryos of radioactive methionine, which presumably gives 

 an indication of the rate of protein synthesis. They found that the in- 

 corporation is particularly rapid in the node, and in the thickened ridges 

 on each side of the streak (Fig. 10.15). There seemed to be some loss of 

 the tracer from the newly invaginated mesoderm, which would suggest 

 that a protein is broken down during the invagination process. The 

 metabohsm of this amino-acid seems to be of considerable importance, 

 since administration of the unnatural analogue etliionine, which would 

 be expected to interfere with the utiUsation of methionine, causes con- 

 siderable inhibition of development. Herrman (1954) has described rather 

 shghter effects of certain other amino-acid analogues. Very marked 

 inhibition, particularly of the streak and of the somites, is also caused by 

 the purine analogue 8-azoguanine (Waddington, Feldman and Perry 

 1955)- This would be expected to affect nucleic acid metabolism, and 

 probably in this way has an influence on protein synthesis, for which 

 RNA is certainly important. 



p This short summary is sufficient to make it clear that there are regional 

 differences of metabolism within the avian blastoderm, and that, in 



