THE AXIAL GRADIENTS IN HYDROZOA. 365 



a fact that has been noticed by us on many different forms. 

 It is to be understood that the disintegration of the ectoderm 

 precedes that of the entoderm by a considerable time interval 

 except in the case of the rim of the mouth where both germ layers 

 seem to disintegrate almost simultaneously forming a great 

 bulging mass of disintegrated particles. 



The remarks in the foregoing paragraph refer to fully developed 

 hydranths. In young hydranths, it was commonly observed that 

 the distal tentacles disintegrated first and the disintegration then 

 extended basipetally along the body of the hydranth, the. proximal 

 tentacles disintegrating later than the body of the hydranth. 

 In medusa-buds, the disintegration proceeds from the free to the 

 attached end. Young hydranths are more susceptible than fully 

 developed ones only after they have reached a certain stage. 



The hydranths are much more susceptible than the stems. 

 The disintegration of the stems is obscured by the presence of the 

 perisarc. Nevertheless the progress of disintegration was ob- 

 served in many cases in the stems but was never followed for 

 more than a short distance. The disintegration proceeds from 

 the base of the hydranth down the stem. It might be said in 

 criticism that the killing agent can gain access to the stem only 

 from the open top of the perisarc and that the death gradient in 

 the stem may therefore be simply a consequence of the diffusion 

 path followed by the agent. To answer this objection short 

 apical pieces of stems bearing hydranths were cut off and their 

 disintegration followed in cyanide. In such cases the open cut 

 surface at the proximal end of the stem affords a readier point 

 of entrance for the reagent than does the top of the perisarc; 

 nevertheless except for a small area of disintegration around the 

 cut surface, the disintegration proceeds in the same manner as 

 before, from the distal end of the stem proximally. 



Direct observation of the course of disintegration in the stems 

 is not, however, entirely satisfying, as the process is admittedly 

 obscured by the presence of the perisarc and could be observed 

 for only the most distal region of the stems. Another method 

 was therefore employed, namely, the differential survival of distal 

 and proximal pieces of the stem in nearly lethal concentrations 

 of toxic substances. The substance employed for this purpose 



