354 



METAMORPHOSIS 



to designate the reproductive organs of the lower plants as spores, so that we 

 may describe these motile spores as ' swarmspores '. Such a swarmspore after 

 a few hours' motility comes to rest, throws its cilia off, envelops itself in a cell- 

 wall, and grows into a new cell-filament. Another type of swarmspore also 

 occurs to which the name of gamete is given. These swarmspores are dis- 

 tinguished from those already described not only by the fact that they are 

 smaller and possess only two cilia {E) but also especially by their subsequent be- 

 haviour. After escaping from the mother-cell \D) they come together in pairs 



and fuse with each other, forming the 

 so-called zygotes (F-H). The zygote as- 

 sumes a thick investment (/), and, after 

 a long resting period, germinates, giving 

 rise to four ordinary swarmspores {K). 

 Let us now compare the reproduc- 

 tive phenomena as seen in Ulothrix with 

 those of the freshwater alga, Oedo- 

 gonium. Here also we meet with a 

 cellular filament fixed at one end to the 

 substratum, also exhibiting two types of 

 reproduction, i. e. by swarmspores and 

 by fusion of cells. The swarmspores in 

 this case originate, as a rule, one in each 

 cell,'sothat the whole of the cell-contents 

 goes to form one swarmspore ; in this 

 case no increase in number is associated 

 with reproduction. It is of no conse- 

 quence for our present purpose that the 

 swarmspores of Oedogonium are struc- 

 turally different from those of Ulothrix, 

 all that we need lay stress on is the fact 

 that, as in Ulothrix, after a certain time 

 the movement in the swarmspore ceases 

 and a new cell-filament is formed. So far 

 as the other method of reproduction is 

 concerned considerable difference exists. 

 In Oedogonium the two cells which unite 

 to form the zygote are quite different 

 from each other, they originate in differ- 

 ent cells, and often in different filaments. 

 The contents of certain cells, distin- 

 guished by their greater size, and 

 known as oogonia, contract, and the 

 cell-wall develops a special opening 

 (Fig. io6). The cell-contents do not 

 escape, however, but fuse in situ with 

 another cell which enters into it through 

 this opening. These other cells have the form and power of movement 

 of the ordinary swarmspores, but differ from them in their smaller size, 

 reduced amount of chlorophyll, and their mode of development (Fig. io6, 

 ///). The large cell is known as the ' ovum ', the small cell as the ' sperma- 

 tozoid '. After these two cells have fused, the zygote, or, as it is here termed, 

 the ' oospore ', becomes surrounded by a thick membrane, passes through 

 a hibernating period, and then germinates, giving rise to ordinary swarmspores. 

 The terminology we have used, viz., ovum and spermatozoid, indicates that we 

 have here to do with a sexual act comparable with that met with in the higher 

 animals. The ovum is the female, the spermatozoid the male element, and it 



Fig. 105. Ulothrix zonata. A, young filament 

 with rhizo'idal cell, r ( X 300). B, portion of a filament 

 with escaping swarmspores, two ' . " ^ 



each cell. 



a swarmspore. /?, formation of gametes and emptymg 

 of a portion of the filament. E, gametes. /% <>, con- 

 jugation of gametes. H, zygote. _/, zygote after the 

 end of the resting period. A, zygote whose contents 

 have divided into swarmspores. B-K^ x 482. After 

 DODEL-PORT, (from the Bonn Textbook). 



