The increase of the central capsule by division was first described in 1862 in my Monograph (L. N. [16], p. 146); since then R. Hertwig (L. N. [26], p. 24) and K. Brandt (L. N. [52], p. 144) have confirmed my statement. In the Phæodaria the division of the central capsule appears always to take place in the main axis; in the bilateral sometimes in the sagittal, sometimes in the frontal plane. In the Tripylea each daughter-cell seems to retain one parapyle and half the astropyle (compare the general description of the Phæodaria, Pl. [101], figs. 1-6, Pl. [104], figs. 1-3, and also Hertwig, L. N. [33], p. 100, Taf. x. figs. 2, 11). Regarding the spontaneous division of colonies of the Polycyttaria, see K. Brandt, L. N. [52], p. 142.
214. Cell-Gemmation.—Reproduction by gemmation has hitherto been observed only in the social Radiolaria, but in them it appears to be widely distributed, and in very young colonies is perhaps almost universally present. The gemmules or capsular buds (hitherto described as "extracapsular bodies") are developed on the surface of young central capsules before these had secreted a membrane. They grow usually in considerable numbers, from the surface of the central capsule, which is sometimes quite covered with them. Each bud usually contains a raspberry-like bunch of shining fatty globules, and by means of reagents a few larger or a considerable number of smaller nuclei may be recognised in them; the naked protoplasmic body of the bud is not enclosed by any membrane. As soon as the buds have reached a certain size they are constricted off from the central capsule and separated from it, being distributed in the meshes of the sarcoplegma by the currents in the exoplasm. Afterwards each bud becomes developed into a complete central capsule by surrounding itself with a membrane when it has attained a definite size. From the special relations of the process of nuclear formation, which take place in the multiplication of the social central capsules by gemmation and by cell-division, it would appear that the capsules produced by the former method afterwards produce anisospores, whilst those in the latter way yield isospores (§ [216]).
The gemmules or capsular buds of the Polycyttaria were first accurately described by Richard Hertwig (L. N. [26], pp. 37-39), under the name "extracapsular bodies," and their significance rightly indicated; earlier observers had incidentally mentioned and figured them, but had not seen their origin from the central capsule. Quite recently Karl Brandt has given a very painstaking account of them in the different Polycyttarian genera (L. N. [52], pp. 179-198). In the Monocyttaria such a formation of buds has not yet been observed. The basal lobes of the central capsule, which occur in many Nassellaria, are not buds, but simple processes of the capsule, due to its protrusion through the collar pores of the cortinar septum (§ [55]).
215. Sporification.—Asexual reproduction by the formation of movable flagellate spores has been hitherto observed only in a very small number of genera; but since these belong to very different groups, and since the comparative morphology of the capsule appears to be similar throughout as regards the structure and development of its contents, it may be safely assumed that this kind of reproduction occurs quite generally in the Radiolaria. In all cases it is the contents of the central capsule, from which the swarm-spores are formed, both nucleus and endoplasm taking an equal share in the process; in all cases the spores produced are very numerous, small, ovoid or reniform, and have one or two very long slender flagella at one extremity (see §§ [141], [142]). Since the whole contents of the mature central capsule are used up in the formation of these flagellate zoospores, it discharges the function of a sporangium. The division of the simple primary nucleus into numerous small nuclei, which usually (serotinous Radiolaria) takes place only shortly before sporification, but sometimes (precocious Radiolaria, § [63]) happens very early, is the commencement of the often repeated process of nuclear division, which terminates with the production of a very large number of small spore-nuclei. The nucleolus often divides very peculiarly (§ [69], C). Each spore nucleus becomes surrounded by a portion of endoplasm and usually receives in addition one or more fatty granules, and sometimes also a small crystal (hence the "crystal-spores"). The size of the flagellate zoospores which emerge from the ruptured central capsule and swim freely in the water by means of their flagellum, varies generally between 0.004 and 0.008 mm. The extracapsulum is not directly concerned in the sporification, but undergoes degeneration during the process and perishes at its conclusion.
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). 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 not in the Collosphærida. It is very probable that these two forms of anisospores copulate with each other after their exit from the central capsule and thus produce a new cell by the simplest mode of sexual reproduction. But, since the same Polycyttaria, which produce these anisospores, at other times give rise to ordinary or asexual isospores, it is further possible that these two forms of reproduction alternate with each other, and that the Polycyttaria thus pass through a true alternation of generations. This has not yet been observed in the Monocyttaria, and hence these latter seem to bear to the Polycyttaria a relation similar to that in which the sexless solitary Flagellata (Astasiea) stand to the sexual social Flagellata (Volvocinea). In the two analogous cases the sexual differentiation may be regarded as a consequence of the social life in the gelatinous colonies.
The sexual differentiation of the Polycyttaria was first discovered in 1875 by R. Hertwig, and accurately described in the case of Collozoum inerme as occurring in addition to the formation of the ordinary crystal-spores (L. N. [26], p. 36); compare also the general discussion of Bütschli (L. N. [41], p. 52). Recently Karl Brandt has demonstrated the formation of both homogeneous isospores (crystal-spores) and heterogeneous anisospores (macro- and microspores) in seven different species of Polycyttaria, and has come to the conclusion that in all social Radiolaria there is a regular alternation between the former and latter generations. Compare his elaborate account of the colonial Radiolaria of the Gulf of Naples (L. N. [52], pp. 145-178).
217. Inheritance.—Inheritance is to be regarded as the most important accompaniment to the function of reproduction, and especially in the present case, because the comparative morphology of the Radiolaria furnishes abundant instances of the action of its different laws. The laws of conservative inheritance are illustrated by the comparative anatomy of the larger groups; thus, in the four legions the characteristic peculiarities of the central capsule are maintained unaltered in consequence of continuous inheritance, although great varieties appear in the skeleton in each legion. The individual parts of the skeleton furnish by their development on the one hand and their degeneration on the other, especially in the smaller groups, examples of progressive inheritance. Thus in the Spumellaria the constant formation of the primary lattice-shell (a central medullary shell) and its ontogenetic relation to the secondary one, which is developed concentrically round it, can only be explained phylogenetically by conservative inheritance, whilst on the other hand the characteristic differentiation of the axes in the various families of Spumellaria is to be explained by progressive inheritance. In the Acantharia the arrangement of the twenty radial spines (in accordance with Müller's law, §§ [110], [172]) was first acquired by a group of the most archaic Actinelida (Adelacantha) through hydrostatic adaptation, and has since been transmitted by inheritance to all the other families of the legion (Icosacantha). The morphology of the Nassellaria is not less interesting, because here several different heritable elements (the primary sagittal ring and the basal tripod) combine in the most manifold ways in the formation of the skeleton (compare §§ [123], [124], [182]). The affinities of the genera in the different families yield an astonishing variety of interesting morphological phenomena, which can only be explained by progressive inheritance. The same is true also of the Phæodaria. In this legion the primary inheritance is especially manifested in the constant and firm structure of the central capsule with its characteristic double wall and astropyle, whilst the formation of the skeleton in this legion proceeds in different directions by means of divergent adaptation. The morphology of the Radiolaria thus proves itself a rich source of materials for the physiological study of adaptation and inheritance.