THE EPIGENETICS OF THE EMBRYONIC AXIS 209 



Thus although these studies are as yet in their infancy, they have 

 already yielded considerable evidence that new proteins are being synthes- 

 ised during gastrulation, and it seems likely that immunological methods 

 w^ill be a very pow^erful means of investigating the fundamental problem 

 of protein synthesis during development. 



It will be noted that Clayton found that the gastrula ectoderm contains 

 certain antigens (C and C^, and E and E^) which in the neurula become 

 spatially separated out between the neural plate and epidermis. Her data 

 provide no direct evidence of whether they are already localised in the 

 presumptive areas of the gastrula, but we know that differences in 

 epigenetic behaviour (competence, and capacity for self-differentiation) 

 are only just beginning to arise in gastrulae of this age, and it seems likely 

 that the two groups of antigens, which later become separated, are both 

 present m the same cells in the earlier stage. If this were true, it would be 

 an important fact concerning the mechanism of induction, since we should 

 have to suppose that the antigens characteristic of the neural plate are in 

 some way destroyed or rendered inoperative in the developing epider- 

 mis, and vice versa. Some evidence that this may actually be the case can 

 be found in the important studies of Ebert on the chick. 



Ebert (1950, 1952) first prepared anti-sera against three adult organs, 

 brain, heart and spleen. The most satisfactory method of testing the 

 embryonic stages for reactivity with these was by adding the anti-sera to 

 agar-albumen clots on which the young blastoderms were allowed to 

 develop. At certain critical concentrations of the anti-sera, rather specific 

 effects were produced on the developing tissues. The sera prepared against 

 mesodermal organs (spleen and heart) tended to prevent the differentia- 

 tion of mesoderm even at the primitive streak and early somite stages, 

 while having less effect on the nervous system (except in so far as the 

 latter was affected by the suppression of the inducing mesoderm). The 

 two anti-mesoderm sera differed in that the anti-heart serum had a more 

 drastic effect on the heart than the anti-spleen one. The anti-brain serum, 

 at the critical concentrations, had little effect on the mesoderm but sup- 

 pressed the development of the neural tube. We have then clear evidence 

 that soon after their determination, and during the early phases of differ- 

 entiation, the different tissues produce different antigenicaUy-active 

 chemical substances, presumably protein in nature. 



In later work, Ebert (1953^) used sera prepared against the protein 

 (myosin) of adult chick hearts to test various regions of the early blasto- 

 derm. He found that there was no reaction by blastoderms younger than 

 the mid-streak stage. By that stage, the invagination of mesoderm has 

 got properly under way, and the mesoderm is probably fuUy determined 



