JANUAEY 9, 1914] 



SCIENCE 



75 



gions where the arms were beginning to de- 

 velop. 



Since in Nereis, Chcetopterus and Arbacia 

 the different susceptibilities of different re- 

 gions of the developmental stages make it pos- 

 sible to kill with more or less exactness cer- 

 tain parts of the embryo while other parts 

 may recover and continue development, this 

 method may prove of some value in further 

 investigation of the regulatory capacities of 

 the less active regions when isolated from the 

 influence of the more active. 



The adult forms of a number of species 

 from various groups were examined for a sus- 

 ceptibility gradient. In the hydroid Pennaria 

 tiarella with KCN 0.0025 m. and 0.005 m. 

 such a gradient appears very clearly in the 

 body of the hydranth, death and disintegra- 

 tion beginning at the distal end of the manu- 

 brium and proceeding proximally. A similar 

 gradient exists in the medusa buds of this 

 species. Besides this it was observed that the 

 full-grown hydranths at or near the tips of 

 stem or branches were in general more sus- 

 ceptible than the more proximal. This dif- 

 ference may be due to external factors such as 

 the lower oxygen or higher COj content of the 

 water about the more proximal hydranths in 

 consequence of the greater number of hy- 

 dranths in a given area, but it seems more 

 probable, in the light of various data concern- 

 ing the polarity of plants, that this difference 

 in susceptibility of distal and proximal hy- 

 dranths is the expression of an axial gradient 

 in the colony. 



In several other species of hydroids ex- 

 amined at Woods Hole and at La Jolla, Cali- 

 fornia, among them Tuhularia crocea and 

 Corymorpha palma the gradient in the hy- 

 dranth body is similar to that in Pennaria. 



The ctenophore Mnemiopsis leidyi shows a 

 distinct gradient in susceptibility along each 

 row of swimming plates. The susceptibility 

 of these animals to KCN is very high and 

 most experiments were made with KCN 

 0.0000375 m.— 00005 m. Ehythmic movement 

 of the plates ceases first at the central end of 

 each row, i. e., the end nearest the apical sense 

 organ, and last at the peripheral end. Before 



movement has entirely stopped in the apical 

 region the rhythm of the plates in the periph- 

 eral half or third of the row becomes different 

 from the central rhythm, being usually more 

 rapid and in some cases irregular or periodic. 

 In two cases a perfectly distinct reversal in 

 direction of the impulse was observed at the 

 peripheral end of a row after movement at the 

 central end had ceased. In this case the im- 

 pulse started at the extreme peripheral end of 

 the row and traveled some distance in the cen- 

 tral direction, finally dying out. This con- 

 tinued for an hour or more before movement 

 at the peripheral end ceased. 



This susceptibility gradient is undoubtedly 

 a gradient in the nerve and not in the plates 

 themselves, for the plates do not die in KCN 

 until long after rhythmic movement ceases, 

 and as long as they remain alive direct con- 

 tact stimulation of single plates produces 

 slight movements of the plate stimulated. 

 However, a slight susceptibility gradient does 

 exist in the plates themselves as is evident 

 from the fact that the plates at the central 

 end of the nerve die first and death proceeds 

 peripherally. The time of death is readily 

 determined, for when they die the plates lose 

 their interference colors and become white 

 and opaque. 



As regards the general ectoderm of Mnemi- 

 opsis, it is diificult to determine the time of 

 death accurately, but observations thus far in- 

 dicate that the disintegration of the ectoderm 

 proceeds from the apical region. 



During the course of my observations on 

 susceptibility gradients Dr. Tashiro called my 

 attention to his discovery of a quantitative 

 gradient in CO, production in the claw nerve 

 of the large spider crab, Lihinia canaliculata: 

 this is a long nerve which readily separates 

 into small strands and is therefore favorable 

 for observation of any structural changes 

 which might occur in connection with death 

 in solutions of narcotics. The nerve is mixed 

 but is believed to consist largely of efferent 

 fibers. 



Since there is some evidence in the work of 

 various authors that a gradient of some sort 

 exists in the nerve, the attempt was made to 



