213. _Cell-Division._--Increase by cell-division among the Radiolaria in the early stage, before the formation of the skeleton, is widely distributed (perhaps even general?); in the adults of this class it is rather rare and limited to certain groups. It is most readily observed in the Polycyttaria; the growth of the colonies in this social group depends mainly (and in many species exclusively) upon repeated spontaneous division of the central capsule; all the individuals of each colony (in so far as this has not arisen by the accidental fusion of two or more colonies) are descendants of a single central capsule, which has arisen from an asexual swarm-spore (§ 215) or from the copulation of two sexual swarm-spores (§ 216). Whilst the central capsules of the colonies continually increase by division, their calymma remains a common gelatinous sheath. Among the SPUMELLARIA reproduction by simple cell-division probably occurs also in many monozootic #Collodaria#. Among the ACANTHARIA the peculiar group Litholophida has perhaps arisen by the spontaneous division of #Acanthonida# (see p. 734). Among the PHÆODARIA increase by cell-division seems to occur commonly in many groups, as in the #Phæocystina#, which have no skeleton (Phæodinida, Pl. 101, {cxxxvi}fig. 2), or only an incomplete Beloid skeleton (Cannorrhaphida, Pl. 101, figs. 3, 6, and Aulacanthida, Pl. 104, figs. 1-3). The #Phæosphæria# also (Aulosphærida, Coelacanthida) and the #Phæogromia# (Tuscarorida, Challengerida) appear sometimes to divide; at all events, their central capsule often contains two nuclei. Of special interest is the spontaneous division of the #Phæoconchia#, especially the Concharida (Pl. 124, fig. 6). In all monozootic Radiolaria, the nucleus first divides by a median constriction into two equal halves (usually by the mode of direct division); then the central capsule becomes constricted in the middle (in the PHÆODARIA in the vertical main axis), and each portion of the capsule retains its own nucleus. In the #Phæoconchia# each half or daughter-cell corresponds to one valve of the shell, dorsal or ventral, so that probably on subsequent separation each daughter-cell retains one valve of the mother-cell, and forms a new one for itself by regeneration (as in the Diatoms). In the polyzootic Radiolaria, which already contain many small nuclei, but usually only a single central oil-globule in each central capsule, the division of the latter is preceded by that of the oil-globule. In many Polycyttaria the colony as a whole multiplies by division.