CHAPTER XV 



SYNGAMY 



The sexual union of two gametes is known as syngamy. In the case of 

 organisms having large female gametes and small, more active male 

 gametes, it is customary to refer to syngamy as the fertilization of the egg 

 by the sperm ; but it should be borne in mind that the syngamic reaction is 

 in all cases mutual, and that it is the fusion product of the two gametes 

 which proceeds with development. 



Syngamy has two important results: firstly, it has a pronounced 

 effect upon the physiological state of the cells concerned and hence upon, 

 ensuing developmental processes. This may manifest itself in various 

 ways, commonly in the activation of an egg whose development has 

 ceased temporarily, or in the onset of a dormant condition, as in the 

 zygotes of certain lower plants. Secondly, it brings about a significant 

 alteration in the constitution of the nucleus. Syngamy culminates in the 

 fusion of two nuclei each of which has a gametic chromosome complement, 

 typically a single set; hence the zygote nucleus so formed has a zygotic 

 complement composed typically of two sets. This doubling of the 

 chromosome number (diplosis) has far-reaching consequences, as will be 

 emphasized in later chapters. 



PLANTS 



The general features of gametic fusion in plants are extraordinarily 

 diversified, notably in algae and fungi. ^ 



Algae. — Fusions involving gametes of the various types described in 

 the preceding chapter take place as follows. In Ulothrix two morpholog- 

 ically similar, motile gametes meet and fuse to form a resting zygospore 

 (Fig. 138, A). The exact manner of fusion varies greatly in such forms, 

 but commonly it begins at the ciliated end.^ In Edocarpus, also, the 

 gametes are morphologically similar, but some of them become relatively 

 passive and anchor themselves to the substratum by one cilium, while 



1 For an extensive account of sexual and asexual cell and nuclear fusions in plants, 

 see Tischler (1921-1922). For angiosperms, see also Schlirhoff (1926), Dahlgren 

 (1927a), andSchnarf (1929); for algae, Oltmanns (1922-1923) and G. M. Smith (1933); 

 for fungi, Guilliermond (1913e), Atkinson (1915), Wager (1920), Giiumann-Dodge 

 (1928), and Fitzpatrick (1930). 



2 That such similar gametes are actually unlike sexually, and secrete charac- 

 teristically different substances into the medium, is shown by the work of M. Hart- 

 mann and his associates, chiefly on green algse. See M. Hartmann (1932). 



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