150 PROTOZOOLOGY 



production which occurs universally in the Metazoa. Various types 

 of sexual reproduction have been reported in literature, of which a 

 few will be considered here. The sexual fusion or syngamy which is a 

 complete union of two gametes, has been reported from various 

 groups, while the conjugation which is a temporary union of two 

 individuals for the purpose of exchanging the nuclear material, is 

 found almost exclusively in the Ciliophora. 



Sexual fusion. The gametes which develop from trophozoites, may 

 be morphologically alike (isogametes) or unlike (anisogametes), 

 both of which are, in well-studied forms, physiologically different 

 as judged by their behavior toward each other. If a gamete does not 

 meet with another one, it perishes. Anisogametes are called micro- 

 gametes and macrogametes. Difference between them is comparable 

 in many instances (Figs. 74; 76; 225) with that which exists between 



Fig. 72. a, macrogamete, and b, microgamete of Volvox aureus, 

 XlOOO (Klein). 



the spermatozoa and ova of Metazoa. The microgametes are motile, 

 relatively small and usually numerous, while the macrogametes are 

 usually not motile, much more voluminous and fewer in number. 

 Therefore, they have sometimes been referred to as male and female 

 gametes (Fig. 72). 



While morphological differences between the gametes have long 

 been known and studied by many workers, whatever information 

 we possess on physiological differences between them is of recent 

 origin. Since 1933, Moewus and his co-workers have published a 

 series of papers based upon their extended studies of bacteria-free 

 cultures of many species (and strains) of Chlamydomonas (p. 217) 

 which throw some light on the gamete differentiation among these 

 phytomonadinans. The gametes in Chlamydomonas are mostly 

 isogamous, except in a few forms. Sexual fusion takes place in the 

 majority of species and strains between the gametes produced in 

 different clones, and there is no gametic fusion within a single clone. 

 Moewus obtained "sex substances" from some of the cultures and 

 showed that these are chemotactic substances. Each gamete secretes 

 substances that attract the other and each reacts to the substances 



