DIFFERENTIAL SUSCEPTIBILITY IN POLYCHETES 15 



Hydroides 



A few observations on the susceptibility of the cleavage stages 

 and twenty-four hour trochophores of Hydroides dyanthus 

 showed the same susceptibility relations as in the other forms, 

 namely, in the earlier stages an apico-basal gradient, with apical 

 region showing the highest susceptibility and in the twenty- 

 four hour trochophore two regions of high susceptibility, the 

 apical or anterior (pretrochal) region and the posterior growing 

 region. 



DIFFERENTIAL INHIBITION OF DEVELOPMENT 



In the sea urchin the differences in susceptibility along the 

 axes afford a basis for modifying and controlling development 

 to a very high degree (Child, '16 c) by means of differential 

 inhibition, acclimation and recovery. In the polychetes the 

 possibilities of modification are, as might be expected, much 

 more narrowly Umited. The work of Wilson ('04 a, '04 b) and 

 others has shown that the course of larval development is very 

 definitely and fixedly determined and that apparently but little 

 regulatory capacity exists in molluscs and polychetes. More- 

 over, the earlier larval stages develop within a few hours and 

 with very little growth of anj^ part, and even in the fully de- 

 veloped trochophore, growth is very largely limited to the pos- 

 terior region. Physiologically speaking the polychete egg has 

 apparently attained an advanced stage of specialization of some 

 sort, at least as regards the larval structures, before cleavage 

 begins, and both the number of cell divisions and the amount of 

 protoplasmic growth are very much less than in the sea urchin. 



In consequence of these conditions, not only is the time within 

 which the action of an inhibiting agent may be effective in alter- 

 ing development very short, except as regards the posterior 

 growing region, but the amount of growth under the experimental 

 inhibiting conditions is small, again except in the posterior grow- 

 ing region. In view of these facts it might be expected that the 

 modifications produced in larval development by differential 

 inhibition would be relatively slight, and that there would be 



