Il8 C. M. CHILD. 



branches, i.e., the regions where the susceptibility is on the 

 average lower than anywhere else in the colony. From this 

 region the localization of developing medusa buds and gono- 

 zooids gradually progresses apically in the colony as a whole and 

 in each branch. Moreover, at least many hydranths of Pennaria, 

 after producing medusa buds for a time, degenerate and may 

 be replaced by new ones, but whether such degeneration always 

 follows medusa bud-formation, it is impossible to say. In 

 Obelia the hydranths, which in this form have no direct part in 

 the production of medusae, undergo disintegration or resorption, 

 at least very frequently in regions of the colony basal to the 

 level of gonozooid formation. New hydranths may develop 

 in place of the old, so that this loss of hydranths in relation to 

 gonozooid formation may readily escape notice, unless carefully 

 looked for. 



Evidently the physiological condition which determines the 

 development of medusa buds and gonozooids is attained in the 

 basal regions of the colony earlier than elsewhere, and gradually 

 progresses apically in the colony, followed by a zone in which 

 at least many of the hydranths die or undergo resorption. This 

 region where medusa buds or gonozooids make their first ap- 

 pearance is the region where rate of growth and frequency of 

 hydranth buds has been least and therefore physiological senes- 

 cence has progressed most rapidly. In the sympodial colony 

 of Obelia these regions are the oldest in time as well as physiologi- 

 cally, while in the monopodial Pennaria and Bougainvillea they 

 are the oldest in time except the apical hydranths, but physiologi- 

 cally are the oldest of all, apparently because senescence has been 

 retarded in the apical hydranths by the higher frequency of 

 hydranth reproduction. There can be no doubt that the ap- 

 pearance of medusa buds or gonozooids in a colony is associated 

 with the attainment of a certain stage of progressive development 

 and physiological senescence, and the facts presented above, 

 with their interpretation, throw some light on the problem of 

 this first appearance in basal regions and gradual progression 

 apically in the colony. 



In various alga? of similar growth form to the hydroids the 

 specialized reproductive organs, e.g., the cystocarps in numerous 



