COPULATION 



73 



cases one gamete is only slightly larger than the other, and there is every 

 gradation towards forms like coccidia or malarial parasites, in which the 

 macrogamete is a comparatively large cell and the microgamete a very 

 minute one. 



As an illustration of syngamy in which two ordinary individuals unite, 

 the case of Cercomonas longicauda, as described by Woodcock (1916), may 

 be considered (Fig. 41). Two flagellates of the ordinary type come 

 together and unite by their posterior ends, the union gradually extending 

 forwards. After the two flagellates are completely fused their nuclei 

 unite to form a synkarion. The zygote which is produced may commence 

 multiplying by binary fission in the usual manner, or it may encyst. 

 A similar process occurs in Polytoma uvella, but is modified as a result of 

 the protective covering of the body (Fig. 42). Two flagellates unite by 

 their anterior ends, and the cytoplasmic contents of one flow into the other, 



Fig. 42. — Polytoma uvella: The Process of Isogamy as observed during thS; 

 Course of Three Hours ( x 1,500). (Original.) 



The contents of one flagellate flow into the other, which gradually becomes spherical. Finally, a 

 cyst is formed after complete fusion has occurred. The nuclei were no longer visible in the 

 later stages. The dark rods are the stigmata. 



giving rise to a spherical zygote which becomes encysted. In the case of 

 Copromonas suhtilis two individuals fuse completely, and Dobell (19086) 

 has stated that each nucleus before union gives off two reduction bodies 

 (Fig. 48). All these instances are cases of isogamy, in which the gametes 

 differ little, if at all, from the ordinary adult individuals. Isogamy has 

 been described by Woodcock (1916) for Sj)iromonas angtista and Co'pro7no7ias 

 fuminantium. 



In most cases, however, certain individuals termed gametocytes, which 

 may differ from the ordinary reproducing forms, by a special type of 

 multiplication {(jametogony) give rise to a number of gametes, which then 

 unite. Syngamy of this type occurs amongst non-parasitic Protozoa, and 

 has been described, in the case of Foraminifera [TrichosphcEriii'm), Radio- 

 laria {Thalassicola), and other forms, but the best-known instances occur 

 amongst parasitic Sporozoa. In the reproduction of Monocystis magna, 



