60 GROWTH AND MORPHOGENESIS 



to elucidate it along the lines that have just been discussed. The 

 menace of an unspecific release of a neuralizing substance which is 

 already present in the ectoblast in a masked form, will always loom 

 before the experimenter. The more complex the experiments be- 

 come, the more difficult is their interpretation. For instance, inhi- 

 bition of induction by agents such as ribonuclease, proteolytic en- 

 zymes, etc., may be due to an effect on the ectoblast cells them- 

 selves, rather than to the block of a specific chemical group in the 

 inducing substance. The reacting system, i.e. the ectoblast, may be 

 directly affected by changes in the surrounding medium in two 

 opposite ways: stimulation of neural differentiation (spontaneous 

 neuralization) or loss of competence, which would make the ecto- 

 blast incapable of reacting to inducing stimuli. 



In view of these uncertainties and the difficulty in solving them, 

 another approach must be used; this is why many investigators 

 have preferred to study RNA distribution and metabolism in intact 

 eggs, either placed in normal or experimentally changed conditions. 



3. DISTRIBUTION OF RNA IN NORMAL AMPHIBIAN EGGS 



Because of the quality of the available cytochemical methods, 

 precise observations can be made in the case of RNA distribution 

 during amphibian egg development (Brachet, 1942, 1944). As shown 

 in Fig. 20 (p. 62) a polarity gradient is already visible in the ad- 

 vanced oocytes and in the unfertilized or freshly fertilized eggs. It 

 decreases from the animal to the vegetal pole. At gastrulation 

 (Figs. 2 1 , 22, pp. 62, 63), a secondary RNA gradient decreasing from 

 dorsal to ventral, superimposes itself on the initial animal-vegetal 

 gradient. As a result of RNA synthesis and morphogenetic move- 

 ments the two gradients interact with each other. The outcome is 

 the appearance, in the late gastrula and the early neurula (Figs. 22, 

 and 23, p. 63), of very well-defined antero-posterior(cephalocaudal) 

 and dorso-ventral gradients. The latter is especially apparent in 

 the chordomesoblast. 



When sections of late gastrulae or early neurulae are examined 

 under high power, a high RNA content is found in the space 



Text continued on p. 65 



