THE EPIGENETICS OF THE EMBRYONIC AXIS 2I3 



of these substances iii the cell can be revealed by their fluorescence when 

 illuminated with ultra-violet. It was found that when applied to amphibian 

 gastrula cells they become, in the first place, attached to lipochondria, that 

 is, to cytoplasmic granules which contain considerable quantities of lipid 

 as well as protein. In normal development, as Holtfreter (1946) showed, 

 these granules break down at just the time when the microsomes are 

 increasing in number during gastrulation and it seems likely that the 

 microsomes are, in fact, produced by the dissociation of the lipochondria 

 into their lipid and protein parts. Under the action of the steroid evo- 

 cators the breakdown is accelerated. Finally, Pasteels (195 1, 1953) has 

 noticed a similar appearance of basophilic cytoplasmic granules in gas- 

 trula cells which have been centrifuged and thus caused to undergo spon- 

 taneous neuralisation. 



All these observations provide considerable support for Brachet's 

 suggestion that the microsomes are intimately involved in the reaction 

 of the gastrula ectoderm to evocatory stimuh. They are probably in fact 

 the site of the reactions h to cto d, etc. as postulated above (p. 197). This 

 accords well with the evidence (pp. 90, loi) that centrifugable granules 

 are important in determining the characters of the different regions in 

 mosaic eggs, the gradients in echinoderms, etc. 



Brachet also suggests that microsome-like bodies not only constitute 

 the system which controls the competence of the reacting ectoderm but 

 are also the natural evocator itself, that is to say the substance we symbol- 

 ised as a in the scheme above. He brings forward several pieces of evidence 

 in support of this. Firstly, he claimed (1944) that if dead tissues, capable of 

 evocating, are digested with ribonuclease they lose their inducing power 

 at the same time that the microsomes granules are destroyed; but more 

 recent work (Brachet, Kuusi and Gothic 1952) has shown that this is not 

 necessarily the case ; it may be the protein, rather than the RNA which is 

 effective. Secondly, he points out that in normal development there is a 

 considerable accumulation of basophilic granular material between the 

 invaginated mesoderm and the overlying ectoderm and although it is 

 difficult to be certain, there is some suggestion that these particles are 

 actually passed from the lower layer of tissue into the upper. Thirdly, 

 Brachet stained living mesoderm with a vital dye, neutral red, which 

 attaches itself mainly to the microsome granules. When this mesoderm is 

 placed m contact with reactive ectoderm the induction takes place and at 

 the same time the red colour is seen to pass into the reacting tissue. This 

 might of course be a mere diffusion of the liberated dye itself. That possi- 

 bility is rendered improbable by the observation that if a porous mem- 

 brane, through which the dye molecules can pass, is placed between the 



