The actual union is brought about or facilitated by the well-known phenomenon termed the _danse des sporoblastes_, which is due to various causes. In the case of highly-differentiated gametes (_Pterocephalus_), the actively motile microgametes rush about here and there, and seek out the female elements. In _Stylorhynchus_, Leger has shown that the function of the sterile male gametes is to bring about, by their vigorous movements, the _melee sexuelle_. In the forms where the gametes are isogamous or only slightly differentiated and (probably) not of themselves motile, other factors aid in producing the necessary commingling. Thus in _Gregarina sp._ from the mealworm, the unused somata or cystal residua become amoeboid and send out processes which drive the peripherally-situated gametes round in the cyst; in some cases where the residual soma becomes liquefied (_Urospora_) the movements of the host are considered to be sufficient; and lastly, in _Diplodina_, owing to the extent to which the intertwining process is carried, if each gamete is not actually contiguous to a suitable fellow-conjugant, a very slight movement or mutual attraction will bring two such, when liberated, into contact. An unusual modification of the process of sporoblast-formation and conjugation, which occurs in _Ophryocystis_, must be mentioned. Here encystment of two associates takes place as usual; the sporont-nucleus of each, however, only divides twice, and one of the daughter-nuclei resulting from each division degenerates. Hence only one sporoblast-nucleus, representing a quarter of the original nuclear-material, persists in each half. Around this some of the cytoplasm condenses, the rest forming a residuum. The sporoblast or gamete thus formed is completely isogamous and normally conjugates with the like one from the other associate, when a single zygote results which becomes a spore containing eight sporozoites, in the ordinary manner. Sometimes, however, the septum between the two halves of the cyst does not break down, in which case parthenogenesis occurs, each sporoblast developing by itself into a small spore.