METAGENESIS lOQ 



ferns, where a stage with only a single set of chromosomes (p. 694) 

 alternates with one containing a double set. This condition is found 

 in animals only in the Sporozoa. In view of this diversity of usage, 

 many biologists prefer not to speak of the life-history of Obelia as an 

 alternation of generations, but to use for it the term metagenesis. 



REPRODUCTION AND COLONY-BUILDING 



The above account of the reproduction of hydroids differs in 

 one respect from that which is sometimes given. On the analogy 

 of the budding of Hydra, some authors regard the formation of 

 a hydroid colony by budding as a kind of asexual reproduction 

 in which there are formed numerous ' individuals ' which do not 

 separate. If that is so the alternation of generations contains an 

 indefinite number of acts of asexual reproduction between two 

 sexual acts. We have preferred to treat the polyp stock as one 

 individual containing a number of semi-independent parts — the 

 hydranths — each of which repeats the structure of the whole 

 body as it was at first, and having certain other parts — the 

 blastostyles and most of the coenosarc — which are differently 

 constructed, serve the entire body, and are wholly dependent 

 upon it. This view involves the following considerations. The 

 development of the individual and its reproduction are essentially 

 the same process — morphogenesis, which is also at work in 

 regeneration. Any part of an organism, from the smallest organ 

 to the whole body, is Uable to be repeated, with or without 

 differences between the repeated parts. This phenomenon has 

 been called merism : we have seen it in ciHa, trichocysts, contractile 

 vacuoles, cells, limbs, zooids, etc. Sometimes, as in cilia, cells, 

 or zooids of the same kind, it has not involved differences. 

 Sometimes, as in cells or limbs or zooids of different kinds, it has 

 involved differences between the repeated parts. Sometimes the 

 parts are present in their full number from the first ; sometimes 

 they increase in number as growth goes on. From time to time 

 every organism produces a part which not only repeats its whole 

 structure, but also separates from it by an act of fission. This 

 process is reproduction. In some types of reproduction, as in the 

 budding of Hydra, the repetition of structure takes place before 

 separation. In others, as in the formation of germ ceUs,i the part 



1 Here there may be the additional compUcation that two such separated parts 

 so develop as to produce but one body after fusion. 



