XXXIV THE VOYAGE OF H.M.S. CHALLENGER. 



divides very early into numerous seioarate nuclei, which are usually distributed in the 

 peripheral portions of the central capsule. In the Nassellakia the development of the 

 porochora, and of the podoconus which stands upon it, brings about the formation of a 

 vertical axis, and in consequence the central capsule assumes a monaxon form {usually 

 ovoid or conical) ; the nucleus then lies in the main axis, but excentrically between the 

 apex of the j)odoconus and the aboral pole. In many Nassellaeia, however, especially 

 when the jDodoconus is so large that its apex approaches the aboral pole of the central 

 capsule, the nucleus is pressed to one side and lies quite excentrically. The Ph.'EODaeia 

 exhibit a different arrangement ; the large spheroidal nucleus is always subcentral, so 

 that its main axis corresponds with that of the concentric spheroidal central capsule ; but 

 since the astrojiyle always occupies the oral pole of the latter, and since the distance of 

 the nucleus from this pole is always somewhat different from its distance from the other, it 

 follows that, strictly speaking, the nucleus never lies accurately in the geometrical centre. 



65. Homogeneous and Allogeneous Nuclei.- — The nucleus of the Eadiolaria 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, Spumellaeia, and that both in many Monozoa 

 (especially smaU Sphseroidea and P r u n o i d e a) and in the Polyzoa (or Polycjrt- 

 taria). 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, hsematoxyline, &c. They may be readily distinguished by 

 these means from the clear vacuoles or "hyaline vesicles," which are evenly distri- 

 buted in the endoplasm of many Eadiolaria, 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 Eadiolaria. 

 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. Tlie Form of the Nucleus. — The nucleus of the Eadiolaria shows greater 

 variations in form and structure than are to be found in the majority of cell-nuclei ; 



