DIFFERENTIAL SUSCEPTIBILITY IN POLYCHETES 13 



usually die before any other part of the ectoderm, but only after 

 they have formed the prototroch. In later stages, and par- 

 ticularly after the deposition of pigment begins, the susceptibility 

 of the trochal region decreases rapidly until, in the more advanced 

 larvae, it is the least susceptible region of the ectoderm. Ap- 

 parently the trochal cells attain their maximum physiological 

 activity or physiologically youngest stage very early and after 

 that undergo rapid senescence. 



After it has attained its definitive condition, the prototroch 

 region stains with neutral red more rapidly than other parts of 

 the ectoderm, but like other ectodermal regions, gradually 

 loses its capacity to hold the dye when subjected to the action 

 of the various agents used. 



Arenicola 



The earlier stages of development have not been available for 

 susceptibility determinations, for none of the egg strings col- 

 lected showed anything earlier than the first motile stages, and 

 I have never yet succeeded in making fertilizations with eggs 

 and sperm from the body cavities of the animals. 



In the stages available the susceptibiUty relations are similar 

 to those in Chaetopterus and Nereis. In the earliest motile 

 stages posterior elongation is already beginning and the somatic 

 plate region is usually the most susceptible part of the ectoderm, 

 the apical region slightly less so and the trochal and intermediate 

 region least susceptible. In the three-segmented trochophore 

 the same relations persist. Death usually appears, first in the 

 ectoderm of the posterior growing region and slightly later 

 in the pretrochal region, though in some cases it begins at the 

 same time in both. From the posterior growing region death 

 progresses anteriorly, in the order third, second, first body 

 segments, and the trochal region is the last portion of the ecto- 

 derm to die. Mesoderm dies later and entoderm much later 

 than ectoderm. 



The Arenicola larva undergoes complete metamorphosis in 

 the laboratory without any external source of nutrition. Young 

 worms with five or six body segments can be obtained in large 



