THE EPIGENETICS OF THE EMBRYONIC AXIS 211 



entirely from the anterior region of the blastoderm. By the next stage 

 (head-fold and early neural groove) it has disappeared also from the anter- 

 ior part of the streak, and is clearly becoming confined to the actual heart 

 rudiments. 



This beautiful series of experiments seems to provide clear evidence 

 that a specific protein can be synthesised throughout the whole of an 

 embryonic region (here the presumptive mesoderm) and then, as regional- 

 isation proceeds within the region, disappear, or at least become un- 

 detectable, in all parts except the actual rudiment of the particular organ 

 to which it belongs. It might perhaps be that its disappearance is illusory, 

 and that it merely becomes concealed because it fails, outside its own rudi- 

 ment, to increase as fast as it does inside it, or as fast as other substances 

 are doing. But the evidence suggests that this is improbable; it seems more 

 likely that the disappearance is a real one. If so, the fact is of capital im- 

 portance for our understanding of epigenetic processes. It seems quite 

 compatible with the hypothesis (advanced on p. 407) that in develop- 

 ment we have to deal with systems of competing processes of such a kind 

 that if one of them gets an initial advantage in a group of cells it will 

 eventually run away with the whole system in that region. Provided that 

 the synthetic processes are reversible, it would be possible for a substance 

 A to be produced at an early stage, and then, as some other process leading 

 to the formation of B pulled ahead, for the material which had gone into 

 the A channel to be as it were sucked back and taken over by the B process, 

 so that A disappeared again. 



Another immunological investigation which has pushed back the 

 detection of specific substances to an early stage, is that of ten Gate and 

 van Doorenmaalen (1950). Using anti-sera against chick and frog adult 

 lenses, they could detect a reaction with the embryonic lens at the stage 

 when it is first becoming detectable anatomically as a vesicle or epidermal 

 thickening. 



Clayton (1954^ and Clayton and Feldman (1955) have recently intro- 

 duced a new refinement into the techniques for recognising embryonic 

 proteins by the use of antisera. The antibodies are coupled with either 

 a fluorescent dye or radio-active iodine. When they are then allowed 

 to act on sections of frozen-dried material, the position of the dye or of 

 the radio-active material (which can be made visible by autoradiography) 

 indicates the location of the corresponding antigens. 



Besides these direct attacks by new teclmiques on the problems of pro- 

 tein synthesis in embryos, a considerable amount has been learnt about 

 evocation by older methods. Brachet (1944, 1952^, b) has drawn attention 

 to the importance of small granules which can be seen in the cytoplasm 



