In the vast majority of Gregarines, however, the life-cycle is limited to gametogony and sporogony. A very general, if not indeed universal, prelude to gametogony is the characteristic and important feature of the order, known as association, the biological significance of which has only lately been fully brought out (see H. M. Woodcock [31]). In normal association, two individuals which are to be regarded as of opposite sex, come into close contact with each other and remain thus attached. The manner in which the parasites join varies in different forms; the association may be end-to-end (terminal), either by like or by unlike poles, or it may be side-to-side (lateral) (fig. 12). The couple (syzygy) thus formed may proceed forthwith to encystment and sporoblast-formation (_Lankesteria, Monocystis_), or may continue in the trophic phase for some time longer (_Gregarina_). In one or two instances (_Zygocystis_), association occurs as soon as the trophozoites become adult. This leads on to the interesting phenomenon of precocious association (neogamy), found in non-motile, coelomic Gregarines (e.g. _Cystobia_, _Diplodina_ and _Diplocystis_), in which the parasitism is most advanced. Woodcock (_loc. cit._) has described and compared the different methods adopted to ensure a permanent union, and the degree of neogamy attained, in these forms. Here it must suffice to say that, in the extreme condition (seen, for instance, in _Diplodina minchinii_) the union takes place very early in the life-history, between individuals which are little more than sporozoites, and is of a most intimate character, the actual cytoplasm of the two associates joining. In such cases, there is absolutely nothing to indicate the "double" nature of the growing trophozoite, but the presence of the two nuclei which remain quite distinct. [Illustration: From Wasielewski, after Leger. FIG. 11.--_Eirmocystis spp._ a, b, Associations of two and three Gregarines; c, Chain of five parasites; p, Primite; s, Satellites.] There can be little doubt that, in the great majority, if not in all Gregarines, association is necessary for subsequent sporulation to take place; i.e. that the cytotactic attraction imparts a developmental stimulus to both partners, which is requisite for the formation of primary sporoblasts (gametes). This association is usually permanent; but in one or two cases (perhaps _Gonospora sp._) temporary association may suffice. While association has fundamentally a reproductive (sexual) significance, in some cases, this function may be delayed or, as it were, temporarily suspended, the cytotactic attraction serving meanwhile a subsidiary purpose in trophic life. Thus, probably, are to be explained the curious multiple associations and long chains of Gregarines (fig. 11) sometimes met with (e.g. _Eirmocystis_, _Clepsydrina_).