PROTOZOA 33 



the time of gamete formation, and their gametes unite each with one 

 from the other parent. Hermaphroditism appears in the CiUophora^ 

 (except the Opalinidae and Chonotricha). Here congress (Fig. 26) 

 takes place between two individuals (conjugants) in each of which the 

 meganucleus (see above) disintegrates, and the micronuclei divide to 

 form a number of nuclei — perhaps a reminiscence of the formation 

 of numerous merogametes. All but one of these nuclei disappear, 

 and the survivor divides to form a male pronucleus, which passes over 

 into the partner, and a female pronucleus which, in possession of the 

 cytoplasm of the parent, awaits the arrival of the male pronucleus of 

 the partner. Fusion now takes place between the male and female 

 pronuclei in each of the pair of conjugants, the latter separate, and 

 by the division of their zygote nuclei mega- and micronuclei arise. 

 Two hermaphrodites have formed each a male and a female gamete 

 and cross-fertilization has taken place .^ In the Vorticellidae (Fig. 26 B) 

 the individuals which enter into congress differ, one being of the 

 ordinary size and fixed, the other small and free-swimming. The 

 smaller arises from an ordinary individual, as a bud or by repeated 

 fission. After reciprocal fertilization of the type just described, the 

 smaller partner perishes, its endoplasm being sucked into the larger. 

 This curious simulation of sexual dimorphism by hermaphrodites 

 occurs in a less marked form in other ciliates. 



A remarkable process known as autogamy, in which a nucleus 

 divides into two which after maturation immediately reunite, occurs 

 in Actinophrys and Actinosphaerium (see pp. 83-86), and possibly in 

 some other cases. 



Parthenogenesis is known to occur in members of at least three of 

 the four classes of the phylum. The clearest case is presented by 

 Actinophrys, when gametes which have failed in attempt at cross- 

 fertilization develop parthenogenetically (p. 86): it is interesting that 

 one of these gametes is a functional male. Individuals of Polytoma 

 which are potential gametes will grow and divide, and the same is 

 true of the gametes of some species of Chlamydomonas and Haema- 

 tococcus when syngamy has been missed. The endomixis of ciliates 

 (p. 35) is a phenomenon of this kind. 



Since it is comparatively easy to observe the conditions which pre- 

 cede and the results which follow syngamy in the Protozoa, many 

 experiments and observations have been made upon those creatures, 

 with a view to discovering the signifi^cance which the process has for 



^ Actinophrys (p. 83) may be said to be hermaphrodite, and so perhaps are 

 many of the Radiolaria. But it is not certain that the " gametes " of this group 

 are not parasitic dinoflagellates. (See p. 80.) 



^ Occasionally (Collinia, Dendrocofuetes) the conjugants also exchange 

 halves of their meganuclei. The latter, however, always disintegrate. 



