74 



SCIENCE 



[N. S. Vol. XXXIX. No. i 



of the body to certain narcotics and poisons, 

 KCN, alcohol and ether being chiefly used. 

 To concentrations of these and various other 

 substances which kill within a few hours with- 

 out permitting any acclimatization the suscep- 

 tibility varies in general with the rate of meta- 

 bolism, or of certain fundamental metabolic 

 processes, i. e., the higher the rate of these 

 processes the greater the susceptibility and 

 the earlier death or cessation of movement oc- 

 curs.- Death in these reagents is usually fol- 

 lowed very soon, often almost at once, by 

 rounding, separation or disintegration of the 

 cells, so that the time of death can be approxi- 

 mately determined by visible changes of this 

 kind. Results obtained in this manner can be 

 controlled by removing the animals from the 

 solution at different periods and determining 

 when recovery ceases to occur and experience 

 has shown that these two methods of proced- 

 ure give essentially similar results. In this 

 way the following forms were examined. 



In Nereis virens the regional susceptibility 

 of developmental stages from the beginning 

 of cleavage to the late trochophore was deter- 

 mined. In the early cleavage stages the 

 micromeres are more susceptible to KCN 

 0.005 m. than the macromeres. They not only 

 disintegrate before the macromeres when the 

 eggs remain in the solution, but if the eggs 

 are returned to sea water at the proper time 

 the micromeres alone are killed and the 

 macromeres recover and resume division, giv- 

 ing rise to defective larvae. 



At the stage when gastrulation is nearly 

 completed the somatic plate region is appar- 

 ently the most susceptible region of the em- 

 bryo, and by return to water at the proper 

 time it ■ is possible to obtain larvae which do 

 not elongate posteriorly and do not form 

 the three larval segments. If the embryos 

 at this stage are left for a longer time in 

 KCN before return to water, both somatic 

 plate and some or all of the macromeres 

 are killed and the intact portion consists 

 of more or less of the ventral portion of pre- 

 trochal and post-trochal ectoderm with or 



2 Child, Jour. Exp. Zool., XIV., 1913. 



without a part of the macromeres. Evidently 

 the most susceptible regions at this stage are 

 first the somatic plate, and second, the dorsal 

 part of the pretrochal region and the macro- 

 meres. 



In the developing egg of another annelid, 

 Chwtopterus pergamentaceus, the relative sus- 

 ceptibilities of different regions are much the 

 same. In the early stages the animal pole 

 shows the highest susceptibility and in later 

 stages a second region of high susceptibil- 

 ity appears in the somatic plate. In still 

 another poly chaste, Arenicola cristata, the 

 apical region and somatic plate of the young 

 trochophores are the most susceptible regions. 

 The early cleavage stages of this species were 

 not obtained. 



In Nereis and Chwtopterus the region about 

 the animal pole is clearly the region of greatest 

 susceptibility, i. e., of greatest metabolic ac- 

 tivity in the early stages of development. 

 Later the activity in this region becomes rela- 

 tively less in Nereis as differentiation of the 

 apical larval region advances and the somatic 

 plate becomes the most active region of the 

 egg. But in Ghwiopierus the apical region 

 retains its susceptibility to some extent at the 

 completion of gastrulation, and this region 

 and the somatic plate appear as distinct re- 

 gions of high susceptibility. In other words, 

 at the beginning of development an axial 

 metabolic gradient exists with the region of 

 highest rate about the animal pole, but as de- 

 velopment proceeds this gradient is altered 

 from its primary simple form by the increase 

 in activity of the cells which give rise to 

 body segments and later by decrease in activ- 

 ity in the animal pole region. 



In the egg of the sea urchin Arhacia in 

 KCN 0.005 m. a distinct susceptibility gradi- 

 ent was observed during cleavage, death and 

 disintegration beginning at one region of the 

 egg and proceeding along an axis, but it was 

 not possible to determine whether the region 

 of highest susceptibility was always the animal 

 pole, though in many cases it certainly was. 

 In the later gastrula and prepluteus stages 

 this simple gradient was complicated by the 

 appearance of high susceptibility in the re- 



