Ulotrichacex 



285 



with four cilia and microzoogonidia with two cilia. Moreover, all the 

 gradations of zoogonidia may be produced from the cells of the same 

 filament. In U. zonata 1G or 32 microzoogonidia are produced from each 

 mother-cell, but only 2 or 4 in U. subtilis; similarly, 2, 4 or 8 macro- 

 zoogonidia (often complanate) are formed in U. zonata, but only one in 

 U. tequalis and U. subtilis. The zoogonidia germinate directly on coming 

 to rest, and sometimes while still within the mother-cell (fig. 183 E and 0). 

 Owing to the great variability in the size of the zoogonidia of U. zonata, the 

 filaments of this species vary greatly in thickness, but in other species of 

 the genus there is a much greater uniformity in the thickness of the 

 filaments. 



F 



Fig. 182. A E, Ulothrix zonata (Web. & Mohr) Kiitz. A, formation of macrozoogonidia ; 

 5, macrozoogonidium showing 4 cilia, pigment-spot (st) and pyrenoids (py) ; C, formation 

 of microzoogonidia; D, microzoogonidium ; E, escape of gametes, their conjugation (g), 

 and the zygote some days after its formation (z). F, the zoogonidia of a form of U. tenerrima 

 Kiitz. All x800. 



Gamogenesis takes place by the fusion of iso- or anisogametes. The 

 gametes are always biciliated, and in some species of Ulothrix and Sticho- 

 coccus they are sufficiently differentiated in size to be termed macro- and 

 microgametes. They may conjugate isogamously or anisogamously, or 

 germinate without conjugation (parthenogenetically). The gametes are 

 frequently quite indistinguishable from the microzoogonidia, a fact which 

 causes one to suspect that the latter are facultative gametes. The zygote 

 becomes invested with a thick wall and undergoes a more or less extended 

 period of rest. On germination it becomes converted into a unicellular 

 germ-plant, which gives rise to 8 or 16 swarm-spores (zoogonidia). 



The conditions necessary for the production of zoogonidia in Ulothrix zonata were 

 investigated by Klebs ('96), but most of his conclusions have not been verified, even for 



