FIELDS AND GRADIENTS IN NORMAL ONTOGENY 349 



apical structures. This has been confirmed by first of all finding 

 the most susceptible region of Anuran gastrulae, then, in another 

 experiment, staining this region intra vitartiy and subsequently 

 producing cyclopia with LiCl and finding that the stained region 

 gives rise to the prechordal part of the brain. ^ A similar explana- 

 tion will apply to cyclopia in regenerating Planarian heads (p. 301). 



Fig. 168 

 Effect of lithium chloride, applied for about 24 hours during late gastrulation, on 

 the mouth region of Bufo vulgaris, (i) Mouth of normal larva, showing horny 

 beak {b.), rows of horny teeth, and lateral papillae (p.a.). (2-4) Mouth of lithium 

 larvae ; (2) showing fusion of the two parts of the beak across the aperture, and great 

 lateral compression ; (3) and (4), mouth reduced to two or one projections, in some 

 cases without horny teeth ; no beak. (Redrawn after Cotronei, Riv. Biol, iii, 1921 .) 



In such poikilothermal systems it is clear that the action of the 

 gradient-fields cannot be concerned solely wdth the absolute values 

 of some fundamental process such as oxidation, but with something 

 more complex, involving primarily the relative values at different 

 points — in other words, with the form of the gradient rather than 

 the absolute intensity of the processes constituting it. 



^ Guareschi, 1932. 



