il8 FRESH-WATER BIOLOGY 



as in Ankistrodesmus, Dactylococcus, and Chlamydomonas, but obser- 

 vation proves that such divisions are always transverse or longi- 

 tudinal, and that the parts in growing slip by each other and 

 elongate, producing the diagonal line of demarcation between 

 them. 



In reproduction by internal division, the new individuals are 

 called by Artari gonidia, by West autospores, and by Wille akin- 

 etes, with the akinete character but sKghtly developed. The 

 contents of such cells may become denser, and possibly be filled 

 with oil or starch; at the same time the membrane becomes thick- 

 ened and the whole cell more resistant to unfavorable conditions, 

 such as heat, cold, or drought. They may remain in this condi- 

 tion for long periods, and in this way maintain the life of the 

 organism over conditions which would threaten the existence of 

 an ordinary vegetative cell. Such cells or akinetes, according to 

 Wille, may be seen in the palmella condition of Stigeoclonium and 

 Chaetophora. 



The modification of these cells may continue farther, and a 

 rejuvenescence occur. Each cell becomes invested with a new 

 membrane and the old membrane is cast off before germination. 

 These structures WUle would designate as aplanospores. He also 

 calls attention to the fact that there are many transitional stages 

 between the vegetative cells and akinetes, and also between the 

 akinetes and the aplanospores. 



In many of the Confervales and Protococcales, instead of 

 autospores, there are formed motile spores or zoospores. These 

 are mostly oval in shape, without a membrane, with one, two, or 

 four ciha, a reddish pigment spot, one or two chromatophores, 

 and usually two contracting vacuoles in the anterior end. The 

 zoosporangiiun, or cell in which they are borne, is in the greater 

 number of cases developed from an ordinary vegetative cell, but 

 more rarely from a cell specialized for that purpose. The zoo- 

 spores originate by the repeated bipartition of the cell contents, by 

 which 2, 4, 8, i6, 32, 64, or even 128 spores are formed, as in the 

 production of autospores. More rarely a single spore is formed 

 from a cell. The zoospores are set free either by the entire cell 

 wall becoming gelatinous, or by its dissolving at a single point, 



