C. The peculiar mode of nuclear budding, by which these small nuclei arise, appears to proceed in the following manner (Pl. [129]). The vesicular primary nucleus, which, in consequence of the centrogeneous development of the skeleton protrudes as it grows into irregular lobes (Pl. [129], fig. 9), assumes a peculiar concavo-convex form, sometimes that of a hood or dish, sometimes that of a kidney or sausage. The convex surface is apposed to the capsule-membrane, while the concave is turned towards the central star of the skeleton (fig. 6). There is now formed at the centre of the convex surface of the strong, doubly-contoured, nuclear membrane, a flask-shaped invagination with a narrow neck and expanded base; the membrane now becomes disposed in peculiar folds, which at the narrow aperture of invagination appear as folds, but on the expanded body of the flask take the form of concentric rings, laid closely side by side (Pl. [129], fig. 10). The convex bottom of the flask, which is directed towards the concave proximal side of the nucleus, becomes again invaginated by a central conical apophysis of the enlarged nucleolus, which is situated between them. Usually the nucleolus has already become flattened into a lentiform shape, and upon its distal face a conical apophysis has been developed, which is divisible into a darker proximal and clearer distal portion. The tip of the latter appears to be in direct connection with the nuclear membrane at the centre of the base of the flask-shaped invagination (figs. 6, 10). At this stage of development the nucleus of the Acantharia generally presents the characteristic form of a hood-shaped, concavo-convex vesicle, whose radial axis is also the axis of the flask-shaped distal invagination, and of the depressed conical nucleolus, which lies between the latter and the concave side of the nucleus. After this peculiar invagination has persisted for some time in connection with the enlarged nucleolus, both disappear, and then a remarkable growth of lobular processes takes place on the concave proximal side of the hood or kidney-shaped nucleus; from four to eight knobs of unequal size usually appear, and their thickened wall encloses a variable number of small of nucleoli; these are at first few but afterwards more numerous (fig. 7). Subsequently these knobs or lobes become completely separated by constriction from the original central mass of the nucleus, and appear as so many separate independent "sausage-shaped bodies" in the hollow central capsule (fig. 8). Each of the bodies now appears, and at first on its convex aspect, to form a large number of small nucleoli, which either separate by constriction from it or become free by its breaking up and lie in numbers in the central capsule. Finally the buds or lobes of the nucleus break up entirely into such nucleoli, which are evenly distributed in the central capsule, and become the nuclei of the swarm-spores (fig. 11). Compare R. Hertwig, L. N. [33], Taf. i.-iii. pp. 19-25.
69. The Nucleus of the Monopylea.—The nucleus of the mature forms of the Nassellaria or Monopylea is generally simple or lobate, homogeneous or vesicular and excentric, and appears only to divide into numerous small nuclei just before the formation of the spores. Nevertheless I have sometimes, though not often, seen in representatives of very various families of the Monopylea, the central capsule filled with many small spherical homogeneous nuclei (Pl. [53], fig. 19). Hence all the families of this legion appear to be serotinous, their simple primitive nucleus persisting for a long period. It is commonly placed excentrically, and most usually in the apical or aboral portion of the central capsule, either between its apex and the podoconus, or quite excentrically on the dorsal aspect. The simple nucleus of the Nassellaria usually appears to be vesicular and to possess a somewhat firm membrane, clear contents, and a rather large, dark coloured nucleolus. In many Nassellaria the nucleus is spherical or ellipsoidal (Pl. [53], fig. 11); whilst in many Stephoidea and Spyroidea, where the central capsule is constricted by the sagittal ring and divided into two symmetrical lateral lobes, the nucleus partakes of the same mode of growth and appears in the middle of the capsule as a transversely placed ellipsoid or even as a short cylinder (Pl. [90], figs. 7, 9). The most remarkable modification in the form of the nucleus is to be found in the multi-articulate Cyrtoidea. Here it is usually enclosed in the cephalis and is spherical, ellipsoidal or spheroidal, often flattened almost into a disc. If now the central capsule increase greatly in size and put forth three or four clavate lobes which hang down through the pores of the cortinar septum into the thorax (or even into the succeeding joints), the nucleus usually undergoes similar modification, and three or four finger-like apophyses are developed from its base, which project into the corresponding lobes of the central capsule (Pl. [59], figs. 4, 12, 13).
The numerous small, spherical, homogeneous nuclei which are to be found in the central capsules of those Nassellaria, which are ripe and about to develop spores, were described in 1862 in my Monograph, as "numerous, small, transparent, spherical cells" in the case of various Cyrtoidea (Arachnocorys, Lithomelissa, Eucecryphalus, Eucyrtidium, &c.) (loc. cit., pp. 302, 305, 309, 321, &c.), and I find them of the same form and dimensions, but deeply stained with carmine in many preparations in the Challenger collection. R. Hertwig has delineated them very accurately in the case of Tridictyopus (1879, loc. cit., p. 84, Taf. vii. fig. 3). He was also the first to recognise the uninucleate condition of the Nassellaria, which is much more frequently observed than the serotinous multinucleate condition, and he described very clearly the peculiar lobed nuclei which arise in Cyrtoidea, owing to the protrusion of the nucleus through the cortinar septum (loc. cit., p. 85, Taf. viii. figs. 3-8).
70. The Nucleus of the Cannopylea.—The nucleus presents the same remarkable structures in all species of the Phæodaria or Cannopylea which have been examined, and closely resembles the germinal vesicle of an amphibian ovum, being a large spherical or spheroidal vesicle with numerous nucleoli. Its diameter usually amounts to half or two-thirds, sometimes even three-quarters, that of the central capsule. The vertical main axis of the latter is also that of the nucleus, which usually lies somewhat nearer to the aboral pole. The nucleus is generally rather more strongly compressed in the direction of the main axis than the capsule itself. The membrane of the vesicular nucleus is thin, but firm, and encloses a clear or finely granular mass of nuclein. The number and size of the contained nucleoli are variable even in one and the same species, and stand in inverse ratio to each other, an obvious result of the gradual process of division. Commonly from twenty to fifty roundish or spherical, strongly refracting nucleoli, are present; more rarely there are several hundred very small ones. Sometimes the nucleus is penetrated by fine trabeculæ, in whose meshes lie the nucleoli (Pl. [101], fig. 2). In certain nuclei, which contained a few large nucleoli, these were of irregular form, probably the result of amœboid movements (Pl. [101], fig. 1). In the formation of spores in the Cannopylea, the nucleus apparently becomes dissolved, and its numerous nucleoli develop directly into the nuclei or mother-nuclei, which produce the nuclei of the flagellate spores. Furthermore, many Phæodaria seem to multiply by simple cell-division, since very commonly (especially in the Phæocystina and Phæoconchia) two large nuclei (right and left), may be met with in one central capsule; sometimes also a single large nucleus, in which a sagittal constriction marks the commencing division of the capsule (Pl. [101], figs. 2, 36; Pl. [104], fig. 3; Pl. [124], fig. 6, &c.).
The large nucleus of the Phæodaria was first described in my Monograph in 1862, in the case of Aulacantha (p. 263), Aulosphæra (p. 359), and Cœlodendrum (p. 361), as a "large, spherical, thin-walled endocyst," from 0.1 to 0.2 mm. in diameter. More detailed descriptions, especially with respect to the behaviour of the nucleoli were given by R. Hertwig in 1879 (L. N. [33], p. 97).
71. The Endoplasm or Intracapsular Protoplasm.—In all Radiolaria the intracapsular protoplasm, which, for the sake of brevity, may be termed "endoplasm," constitutes originally, and especially in the earliest stages, the only important content of the central capsule, except the nucleus. In certain Spumellaria and Nassellaria, of simple structure and of small dimensions, this condition persists for a long period, and the endoplasm then appears as a homogeneous, colourless, turbid or finely granular, mucous, semi-solid mass, which cannot be distinguished from the ordinary undifferentiated protoplasm of young cells; no definite structure, and in particular, no fibrillar network, can be discovered in it even by the use of the customary reagents. In the great majority of the Radiolaria, however, this primitive homogeneous condition of the endoplasm is very transient, and it soon undergoes definite modifications, becoming differentiated into separate parts or producing new constituent contents. Such products of the internal protoplasm are in particular hyaline spheres (vacuoles and alveoles), oil-globules, pigment-bodies, crystals, &c. The most important of the differentiations which take place in the endoplasm is that into an internal, granular, medullary substance and an external, fibrillar, cortical substance; although the various legions behave somewhat differently in this respect (§§ [77]-[80]).
72. Intracapsular Hyaline Spheres.—The central capsule of very many Radiolaria contains in its endoplasm numerous spherical bodies of varying size, which consist of watery or albuminous fluid, and have previously been regarded as nuclei, or described as products of the internal protoplasm, under various names, such as "spherical transparent vesicles" (see note A, below), "albumen spheres" (see B), "gelatinous spheres" (see C), "alveolar cells" (see D), &c. Some of these spheres are perfectly transparent, structureless and of varying refractive power, producing the impression of drops of fluid; others contain various formed constituents, such as oil-globules, fat-granules, pigment-granules, concretions, crystals, &c. From a morphological point of view they may all be divided into two categories, membraneless vacuoles and vesicular alveoles. The vacuoles are simple spherical drops of fluid or of gelatinous material, devoid of a special envelope, but immediately surrounded by the endoplasm. The alveoles, on the other hand, are true vesicles with a thin spherical envelope, enclosing a drop of fluid or jelly. This envelope is commonly very thin, homogeneous, and often scarcely discernible, so that in practice a sharp line of demarcation cannot be drawn between alveoles and vacuoles; the former are usually somewhat larger than the latter. The fact is, nevertheless, certain that the hyaline spheres, which may be isolated on rupturing the central capsule of many Radiolaria, in certain cases, particularly in large species, possess a clear, anatomically demonstrable membrane, whilst in others no such appearance is presented. It may be assumed that the vesicular alveoles are developed from the drop-like vacuoles by increase in size, and by the precipitation of a delicate envelope from the endoplasm. The character common to all these hyaline spheres, whether vacuoles or alveoles, is found in their aqueous, not adipose, constitution, and in their clear transparent appearance, which allows of no structure (the above-mentioned contained bodies excepted) being recognised. Their refractive power and consistency vary somewhat, and probably their chemical constitution still more. Sometimes they are strongly refractive and shining, and sometimes feebly refractive and pale; their consistency shows all intermediate stages between a thin fluid, which readily disappears in water, and a firm, insoluble jelly. As regards their chemical composition (which is probably very variable), the hyaline spheres may be best divided into two groups, the organic and inorganic. The inorganic hyaline spheres are simple drops of saline solution without any carbonaceous constituent; the organic, on the other hand, contain a small quantity of organic matter dissolved in the watery fluid, and may be either albuminous or gelatinous spheres. The formed contents which are commonly present are of very various natures, usually small fat-granules, more rarely larger fat-granules or pigment-granules, sometimes concretions or crystals. In many groups, especially among the large Phæodaria and Collodaria, the numerous hyaline spheres are remarkable for their equal size and even distribution throughout the endoplasm (Pl. [1], figs. 1, 4; Pl. [104], fig. 2, &c.). In some genera belonging to the Thalassicollida the alveoles are of enormous size (Pl. [1], figs. 2, 3); they then become flattened by mutual pressure into polyhedra and distend the central capsule to unusual dimensions (in Physematium and Thalassolampe 8 to 12 mm.).
A. The "spherical hyaline vesicles," which I described in my Monograph (1862, p. 71) as among the most important and constant contents of the central capsule, are partly vacuoles, partly homogeneous nuclei. Most recent investigators, Bütschli in particular (1882, L. N. [41]), have pointed out and rightly criticised this confusion. The criticism might, however, have been more justly expressed by stating that, in the preparation of my Monograph (1859-1862), I did not make use of modern methods of demonstrating the nucleus by staining fluids, which were quite unknown at the time, and only discovered a decade later. In fact, without the aid of such reagents, it is quite impossible to distinguish between the various "spherical transparent vesicles," of which those found in the central capsule of the Phæodaria and many monozootic Collodaria are simple vacuoles lying in the endoplasm, whilst, on the other hand, those of the Polycyttaria and many other Radiolaria are true homogeneous nuclei. For not only are the general appearance of the small clear spheres, their refractive power, and regular distribution in the endoplasm quite similar, but they are also of much the same size, for the diameter ranges from 0.005 to 0.015 mm., being generally between 0.008 and 0.012 mm. In addition to this there is generally in each hyaline sphere a dark brightly shining granule, which, in the case of the vacuole, is simply a fat-granule, whilst in the case of the nucleus, it is a true nucleolus. The small hyaline spheres in the young uninucleate capsules of the Polycyttaria are simple vacuoles (Pl. [3], fig. 12), whilst in the ripe multinucleate capsules they are true nuclei (Pl. [3], figs. 3, 8, 9), and it is quite impossible to discriminate between these two conditions without the use of reagents. This has been expressly recognised by R. Hertwig, who has the merit of having been the first to clearly distinguish, by the aid of staining fluids, between these two different constituents (1879, L. N. [33], p. 108).
B. The "albumen spheres," which were first observed by A. Schneider in 1858 in the common cosmopolitan Thalassicolla nucleata (L. N. [13], p. 40), and which appear to occur in only a few other Thalassicollida, are distinguished from the ordinary hyaline spheres of about the same size by their higher refractive power and by certain albuminoid reactions, especially the coagulation of a membranous envelope under the influence of certain reagents (see my Monograph, p. 250, and Hertwig, L. N. [26], 1876, p. 46). They often enclose various formed contents, and require further investigation.
C. The gelatinous spheres of various sizes, found in the endoplasm of the Radiolaria, agree in their reactions (especially in staining by certain reagents) with the common extracapsular jelly of the calymma, and are hence distinguishable both from the true (coagulable) "albumen sphere," and from the ordinary watery vacuoles.