332 FIELDS AND GRADIENTS IN NORMAL ONTOGENY 



two gradients, with apical point to the front and hind ends respec- 

 tively. Here, twinning occurs most frequently at each end of the 

 worm, and very rarely in the middle region.^ 



The opposite to twinning is the merging together in the middle 

 line of organs which are typically paired, a good example of which is 

 provided by cyclopia and monorhiny (p. 348). Attention may here 

 be called to the part played in the development of the amphibian 

 eyes by the underlying organiser. It will be remembered (p. 245) 

 that a piece of presumptive eye-region of the neural plate, taken 

 from the middle line without underlying organiser, usually dif- 

 ferentiated into a single eye. The presence of neighbouring under- 

 lying organiser tissue, on the other hand, leads to the development 

 of paired eyes from such grafts. In other words, the organiser has 

 brought about twinning of the rudiments in the field : explanation 

 of this effect is, however, obscure. 



The same principle which underlies twinning by dichotomy in 

 the whole organism can also be applied to the duplication of single 

 organs (see above, pp. 296, 327). 



§ 5 . Differential susceptibility and the modification 

 of ontogeny in invertebrates 



(viii) The effects of differential susceptibility 



Remarkable modifications of normal development have been ob- 

 tained by applying the principles of differential activity to the eggs 

 of Annelid worms.^ In Chaetopteriis , susceptibility experiments 

 show that the animal pole is at first the most active region. This 

 condition persists until the young larva begins to show elongation 

 of the trunk, when the posterior region becomes the most active. 



By immersing the developing eggs in inhibiting agents (e.g. 

 rnj 100,000 KCN) from fertilisation onwards, microcephalic forms 

 are produced. These forms also have their extreme posterior regions 

 inhibited, as the treatment is continued during the period when 

 these show high susceptibility. If the treatment is discontinued 

 after 11 hours from fertilisation, the posterior region is better 

 developed, while the microcephaly persists. 



If, on the other hand, the treatment is not begun until the 



^ Hyman, 1921. ^ Child, 1917, 1925 a. 



