PROTOZOA 31 



knowledge goes at present, by no means universal in the Protozoa. 

 Especially among the Mastigophora, but also in other groups as in 

 the Amoebina, there are many cases in which it appears not to occur. 

 Probably, however, in the majority of species it either is known or 

 may reasonably be inferred to take place. The energids by which it 

 is performed, known here, as in all organisms, as gametes^ may be 

 either merogametes, formed by special acts of fission and smaller than 

 the ordinary energids of the species, or hologametes, not formed by 

 special fissions, and as large as, or larger than, the ordinary energids. 

 Syngamy between like gametes is known as isogamy, that between 

 unlike gametes as anisogamy. The simplest cases of the process are 

 those instances of isogamy in which two full-sized ordinary indi- 

 viduals unite. Such unions are known as hologamy and are rare, 

 though they occur in Copromonas (Fig. 39 F') and a few other species. 

 The fact that nearly all protozoa in which it is certainly known to 

 occur are coprozoic (p. 43) suggests that it is an adaptation to special 

 conditions — perhaps to brief duration of the active stage — and is 

 not, as might be assumed, the primitive form of syngamy. In all 

 other cases the gametes are special individuals, and one at least is 

 a merogamete. They may be isogamous or anisogamous, and in the 

 latter case one — \ht female gamete — is less active than the other, which 

 is the male gamete. Nearly always the female gamete (macrogamete) 

 is larger than the male {microgamete), and often it is a hologamete. 

 In the latter case the process is known as oogamy. As examples of 

 isogamy of merogametes we may cite the syngamy of Polystomella 

 (Fig. 66C-E) and of some Chlamydomonas (e.g. C. steini). 



Anisogamy occurs independently in many genera, and has more 

 than once become oogamy. An interesting series of grades in this 

 respect is provided by the Volvocina. Chlamydomonas euchlora ex- 

 hibits the transition from isogamy to anisogamy. By undergoing 

 different numbers (2-6) of divisions, its individuals form merogametes 

 of several different sizes, but these pair indifferently, some unions 

 being isogamous, some anisogamous. C. brauni and other species 

 (Fig. 25) form merogametes of two sizes and are definitely ani- 

 sogamous. Volvox (Fig. 46) and related forms have an anisogamy 

 in which the female gamete is a hologamete (oogamy). A similar 

 series is shown by the Sporozoa. The syngamy of some species of 

 Monocystis, for instance, is isogamy of merogametes, that of others 

 is anisogamy of merogametes with various degrees of unlikeness 

 between the gametes, and that of the malaria parasite (Fig. 75) and 

 its relations is anisogamy between a hologamete and a merogamete. 



Syngamy, whether isogamous or anisogamous, nearly always is exo- 

 gamous, that is, takes place between the offspring of different parents. 



Since the male and female gametes are usually formed by distinct 



