THE ACTIVATION OF GENES BY THE CYTOPLASM 35I 



attained. Both these points can be expressed visually by means of a diagram 

 such as that of Fig. 16.2, which represents what has been called the 

 'epigenetic landscape' (Waddington 1940a, q.v. for further discussion). 



I. The effects of cytoplasm on the nucleus 



In earlier chapters we have seen several instances in which localised 

 areas of cytoplasm must have exerted an effect on the nuclei which move 



Thora« Wlitf Upp<r kg Lower \tq Antenna Mouth 



Figure 16.2 



The 'epigenetic landscape.' A symbolic representation of the developmental 

 potentialities of a genotype in terms of a surface, sloping towards the obser- 

 ver, down which there run balls each of which has a bias corresponding to 

 the particular initial conditions in some part of the newly fertilised egg. 

 The sloping surface is grooved, and the balls will run into one or other of 

 these channels, finishing at a point corresponding to some typical organ. 

 (From Waddington 1954/7.) 



into them. For example, when any cleavage nucleus comes into the 

 formation centre of the Platycnemis egg, a reaction takes place and a sub- 

 stance is produced which diffuses forwards to the differentiation centre 

 (p. 126). Again any nucleus which reaches the grey crescent region of the 

 amphibian egg can take part in the development of the organisation centre. 

 And all the mosaic eggs similarly show that the role which a nucleus 

 plays in development depends on the type of cytoplasm in which it lies. 

 Rather little is known about the nature of the effects which cytoplasm 



