SUPPRESSION OF MEDUSOID 123 



plant, and again it is the sexual generation which has undergone 

 reduction. 



In other cases the medusoid bud or gonophore more or less 

 completely loses its medusa-like structure and becomes a mere 

 sac containing the gonad, and it has been suggested that in 

 Hydra we may have the last stage in this progressive reduction 

 of the sexual generation, the medusoid having altogether dis- 

 appeared, after having transferred its sexual functions to the 

 hydroid. 



It seems almost incredible that the germ cells themselves 

 which are originally produced by one generation should in this 

 manner be transferred to what is really the preceding generation, 

 but as a matter of fact we see all stages in this process in different 

 genera of Hydrozoa. Even in Obelia and certain other medusae 

 the germ cells do not originate in the gonads beneath the radial 

 canals but in the ectoderm of the manubrium, reaching their 

 final position by a process of migration. In Eudendrium, where 

 the medusoid is reduced to a mere vestige, the germ cells 

 no longer originate in the sexual individual at all, but in the main 

 stem of the hydroid colony, whence they migrate into the 

 medusoid bud. If we imagine them ceasing to migrate from their 

 place of origin in the hydroid, while the medusoid is no longer 

 formed, we fe;ach the condition of Hydra, with a complete trans- 

 ference of the^ sexual function to what should be the asexual 

 generation. 



In the coelenterates, however, it is not always the asexual 



(hydroid) generation which predominates, for in some cases this 



^suppressed more or less completely and the medusa or jelly - 



fish reproduces its own kind directly without the intervention of 



a hydroid phase. 



In coelenterate animals such as Hydra and Obelia, as we 

 have already seen, the body consists of a single hollow tube 

 whose wall is made up of only two cell layers, the ectoderm 

 on the surface and the endoderm lining the digestive cavity, with 

 a supporting layer of gelatinous consistency the mesogloea 

 between the two. In the case of the larger jelly-fish 

 this mesogloea attains a great thickness ; it is, however, 

 never a true cell layer like the ectoderm and endoderm, but 

 rather of the nature of intercellular substance secreted by the 

 cells on either side of it. 



In Hydra and Obelia the germ cells arise from the ectoderm, 



