348 FIELDS AND GRADIENTS IN NORMAL ONTOGENY 



less extent of the apical regions, resulting in animals with eyes in 

 contact, fused eyes, or a single median eye (cyclopia), and a single 

 median nostril (monorhiny). The mouth undergoes corresponding 

 modifications. In Amphibia, the effects may go so far as to give 

 rise to completely eyeless larvae, often with markedly malformed 

 mouths. Neighbouring parts are only very slightly affected, and 

 the trunk region seems not to be affected at all, or to a degree which 

 would be revealed only by precise measurements^ (figs- 166, 167). 

 These curious facts can be explained as the result of differential 

 susceptibility of the different regions of the gradient. A certain 

 level of activity is needed for the formation of apical (anterior) 

 structures, a slightly lower level for those next posterior, and so on. 



Fig. 167 

 Effect of lithium chloride on apical structures in anuran development. Left, 

 control frog tadpole. Right, tadpole from an egg exposed for 3 hours to M/7 LiCl 

 during early gastrulation ; the anterior head region is inhibited, the external 

 nostrils {o.p.) are fused, and the eyes close together. (Redrawn after Bellamy, 

 Biol. Bull. XXXVII, 1919.) 



While chemo-differentiation proceeds apicalwards under the in- 

 fluence of the organiser, the posterior levels of the body can all be 

 determined. But the extreme apical end, being the most susceptible 

 to depressant agents on account of its high rate of activity,- is now 

 in a state which will not permitof the formation of high-level organs. 

 The material of the apical (animal) region is, however, not destroyed, 

 and is used up in the construction of subapical structures. 

 This will explain why certain definite structures are absent from 

 an embryo which has been exposed to depressants in the early 

 stages of cleavage, i.e. long before the structures in question have 

 become determined, let alone differentiated. Another way of 

 putting this interpretation is to say that the whole gradient has been 

 flattened out in such a way that its apical end no longer reaches the 

 threshold potential value needed for the production of extreme 



^ Cotronei, 1921. - See Child, 1915 a; Bellamy and Child, 1924- 



