SEX IN UNICELLULAR ALGAE 119 



Pairing 



In the formation of mating clumps and in the subsequent pairing 

 of d^amctcs, the flaijella of C nioeiviisii play an important role (Lewin, 

 1952b). Mutant strains in which the flagella are lacking (Lewin, 

 1952a) are completely asexual. It is probable that these organelles are 

 equally essential in the sexual processes of other flagellated algae, as 

 are the cilia of PcViT/f/eciir/n and other ciliates (Jennings, 1939). By 

 their aid the mating cells are brought together and held until they pair 

 and cytogamy can be initiated. As soon as pairing has been accom- 

 plished, the agglutination of the flagella ceases, perhaps as a result of 

 a stimulus transmitted through the papillae and basal granules, and the 

 flagella extend free in the medium once more. The whole process 

 occupies less than 10 minutes (at 23°); typical stages are illustrated in 

 Plate I. 



In most unicellular algae, perhaps all, the process of fusion com- 

 mences between the anterior ends of the mating cells. In those in 

 which the gametes are not naked cells, the wall appears to dissolve 

 first in this region, and a minute process may emerge, as in Folytovm 

 (Pringsheim and Ondratschek, 1939), Chlamydovwjias proboscigera 

 (Korschikoff, in Pascher, 1927), and C. eugametos (Moewus, 1933). 

 A thin connecting strand of material, probably protoplasm, has been 

 observed in ChJaviydovwnas and FhyUocardhnn (Korschikoff, 1927), 

 C. praecox (Pascher, 1943), C. chlaviydogavia (Bold, 1949), C. nioe- 

 wusii (Lewin, 1950a; see Plate I, 2-3), and others, connecting the 

 two cells; and ultimately this widens until the protoplasts fuse. 



Although there are several records in the literature of gametes 

 which have begun to fuse at their posterior ends — for example, in 

 Frotosiphon (Klebs, 1896), Dimaliella (Cavara, 1906), Tetraspora 

 (Geitler, 1931), and in Carteria and Chlamydomoiias spp. (Mitra, 

 1950) — there is good reason to believe that in most, if not all, cases the 

 observed cell pairs were not indulging in syngamy but had resulted 

 from incomplete cell division, as pointed out by Lerche (1937). Such 

 twinned cells have been described or figured in Cbkviydovwints 

 brawni (Goroschankin, 1890), C. stellata (Gerloff, 1940), C. varia- 

 bilis (Behlau, 1939), C. iipsalieiisis (Skuja, 1949), C. moeivusii 

 (Lewin, 1952a), Haemato coccus (Reichenow, 1909), Chlorococcmn 

 (Bold, 1930), Frotosiphon (Nayal, 1933), and so on. 



