SUSCEPTIBILITY IN HYDROIDS. 



terpreted wholly in terms of permeability or thickness of perisarc. 

 According to the evidence, the susceptibility relations in tin 

 hydroids are very similar to those observed in the multiaxial 

 thalli of algae (Child, 'i6a, 'i6b, '17), and certain other resem- 

 blances will be pointed out below. 



THE DEATH GRADIENTS IN DEVELOPING MEDUSA BUDS. 



The elongated, naked medusa buds of Pennaria have consti- 

 tuted the chief material for these observations. In the earlier 

 stages of the bud the disintegration of the ectoderm begins at 

 the free, apical end and progresses basipetally (Fig. 4). In 



4 



5 



FIGS. 4-6. Stages of disintegration . in Medusa buds of Pennaria. Fig. 4. 

 Ectodermal disintegration beginning apically in very early bud. Fig. 5. Later bud 

 stage with disintegration beginning on the tentacular outgrowths. Fig. 6. Later 

 stage of disintegration. 



later stages, where the tentacular outgrowths have begun to 

 develop, these disintegrate first of all (Fig. 5), and the proces 

 then progresses basipetally over the body, as in earlier stages 

 (Fig. 6). The entoderm disintegrates somewhat later than the 

 ectoderm and shows, at least in earlier stages, a basipetal gradi- 

 ent. The apical region of the developing medusa bud of Pennaria 

 represents of course the marginal region of the fully grown 

 medusa, the subumbrellar region not being open to the exterior 

 until an advanced stage of development is reached. 



The earlier stages of the gonophores of Tiibularia likewise 

 show a basipetal susceptibility gradient in the ectoderm and a 

 similar, less distinct gradient in the entoderm. The conditions 

 in the fully developed, free swimming hydromedusae will be 

 considered at another time. 



