The first complete and detailed observations on the formation of spores in the Radiolaria were published by Cienkowski in 1871 and related to two genera of Polycyttaria, the skeletonless _Collozoum_ and the spherical-shelled _Collosphæra_ (L. N. 22, p. 372, Taf. xxix.). These were subsequently continued and supplemented by R. Hertwig (1876, L. N. 26, pp. 26-42, and L. N. 33, p. 129), and a general summary of these results has been given by Bütschli (L. N. 41, pp. 449-455). {cxxxviii}Recently Karl Brandt has given a very detailed and fully illustrated account of the sporification of the Polycyttaria (L. N. 52, pp. 145-178). I have also had the opportunity during my sojourn in the Canary Islands (1866), in the Mediterranean at Corfu (1877), and Portofino (1880), as well as in Ceylon (1881), of observing the development of flagellate zoospores from the central capsule of individuals of all four legions: among the SPUMELLARIA in certain #Colloidea#, #Beloidea#, #Sphæroidea#, and #Discoidea#, among the ACANTHARIA in several #Acanthometra# and #Acanthophracta#, among the NASSELLARIA in individuals belonging to the #Stephoidea#, #Plectoidea#, and #Cyrtoidea#, and among the PHÆODARIA in one Castanellid. In most zoospores I could distinctly observe only a single long flagellum; sometimes, however, two or even three appeared to be present, but the determination of their number is very difficult. 216. _Alternation of Generations._--A peculiar form of reproduction, which may be designated "alternation of generations," appears to occur generally in the Polycyttaria, but has not yet been observed in the Monocyttaria. All #Collozoida#, #Sphærozoida#, and Collosphærida which have hitherto been carefully and completely examined with respect to their development, are distinguished by the production of two different kinds of swarm-spores, isospores and anisospores. The _Isospores_ (or monogonous spores) correspond to the ordinary asexual zoospores of the Monocyttaria (§ 215); they possess a homogeneous, doubly refracting nucleus of uniform constitution and develop asexually, without copulation. The _Anisospores_ (or amphigonous spores), on the other hand, are sexually differentiated and possess a heterogeneous, singly refracting nucleus of twofold constitution; they may therefore be distinguished as female macrospores and male microspores. The _Macrospores_ (or gynospores, comparable with the female macrogonidia of many Cryptogams) are larger, less numerous, and possess larger nuclei, which are less easily stained, and have a fine filiform trabecular network. On the other hand the _Microspores_ (or _androspores_, comparable with the male microgonidia) are much smaller and more numerous, and are distinguished by their smaller nuclei, which have thicker tuberculæ and become stained more intensely. The gynospores and androspores are developed in the #Collozoida# and #Sphærozoida# in the same individual, but