52. _The Primary Form of the Central Capsule._--The form of the central capsule is originally a geometrical sphere; and if in accordance with our monophyletic hypothesis all Radiolaria are to be derived from one common stem-form (_Actissa_, see p. 12), then the central capsule of this common stem-form must be regarded as perfectly spherical (_Procyttarium_, p. 13, Pl. 1, fig. 1). Since, further, the enclosed nucleus and the surrounding calymma of this primitive archaic form must also be spheres, and since the nucleus lies in the centre of the body, and the protoplasm is evenly distributed between it and the membrane, it follows that no axes or excentrically differentiated parts are to be distinguished in this most primitive Radiolarian. Rather in the primary central capsule all parts are concentrically and evenly arranged round its centre. This primary spherical form becomes modified in most Radiolaria into various secondary ground-forms, which are correlated partly with the structure of the capsule itself, and partly also with the development of openings in its membrane. In general the ground-form of the central capsule is polyaxon in the Porulosa (SPUMELLARIA and ACANTHARIA); but in the Osculosa centraxon forms are more frequently observed; in the NASSELLARIA the ovoid (allopolar monaxon) form is predominant, and in the PHÆODARIA the rhomboid or amphithect pyramid. In these latter, the astropyle indicates the basal pole of the vertical main axis, whilst the two parapylæ (right and left) mark the poles of the frontal transverse axis. In the NASSELLARIA the centre of the porochora corresponds with the basal pole of the main axis, whilst no transverse axes are originally present. 53. _The Secondary Forms of the Central Capsule._--The original purely spherical form of the central capsule persists only in the minority of the Radiolaria, namely, the greater part of the SPUMELLARIA and ACANTHARIA; it passes over into various other secondary forms in the majority of the class, in the whole of the NASSELLARIA and PHÆODARIA, and in a considerable portion of the SPUMELLARIA and ACANTHARIA. These secondary or derived forms may be divided into two quite distinct groups, which may be designated endometamorphic and exometamorphic; in the former the cause of the divergence of the secondary form from the sphere lies in the internal structure of the central capsule; in the latter it lies in the external influence exerted by the growth of the skeleton. Obviously the former series of modifications is more significant than the latter. 54. _The Endometamorphic Forms of the Central Capsule._--The secondary forms of the central capsule, which are due to internal causes connected with its growth, are as follows:-- {xxvi}A. _The Ellipsoidal Central Capsule_, with one axis elongated, so that it becomes the vertical main axis of the body.