65. _Homogeneous and Allogeneous Nuclei._--The nucleus of the Radiolaria not only exhibits a similar structure and composition, and suffers similar modifications to those which are found to occur in the case of other cell-nuclei, but also to some extent shows very peculiar developmental forms, which are seldom or never found in other cells. In the first place the nuclei may be divided into homogeneous and allogeneous, the former are structureless and consist of a uniform mass of nuclein, whilst the latter are composed of different substances and show various structural relations. _Homogeneous_ nuclei, whose whole mass is uniform and exhibits no structural differentiation, are probably always to be found in the swarm-spores; in the fully developed Radiolarian body they are found only in the first legion, SPUMELLARIA, and that both in many Monozoa (especially small #Sphæroidea# and #Prunoidea#) and in the Polyzoa (or Polycyttaria). The whole mass of these homogeneous nuclei, which are usually spherical or ellipsoidal, consists of uniform, perfectly clear and transparent nuclein, and becomes evenly stained by carmine, hæmatoxyline, &c. They may be readily distinguished by these means from the clear vacuoles or "hyaline vesicles," which are evenly distributed in the endoplasm of many Radiolaria, and may be confused with the former. _Allogeneous_ nuclei, which are always composed of different parts and often show complicated structural relations, are found developed in the great majority of Radiolaria. The most important differentiation exhibited by these secondary forms is the separation of the nuclear mass into a firm nuclear substance (caryoplasm) and a fluid nuclear juice (caryolymph). In addition in each nucleus a nucleolus is visible, and often several or many may be seen (see §§ 67 to 70). 66. _The Form of the Nucleus._--The nucleus of the Radiolaria shows greater variations in form and structure than are to be found in the majority of cell-nuclei; {xxxv}exception must, however, be made in the case of many animal ovicells, which, in their peculiar form and composition, often recall large Radiolarian nuclei. With respect to the external shape two main forms may be distinguished, as primary and secondary. The _primary form_ of the Radiolarian nucleus is the sphere; it occurs not only in most swarm-spores, but also in most adult forms belonging to the legion SPUMELLARIA, and in individual instances in other groups; indeed the nuclei of most SPUMELLARIA, as also the concentric central capsules in which they lie, are true geometrical spheres. The _secondary forms_ of the nucleus are found in the majority of adult Radiolaria, and arise from the primary spherical forms in various ways, either by the elongation or contraction of one axis, or by the formation of apophyses or processes. The most important of these secondary forms are as follows:--