122 



THE RADIOLARIA 



Among the recently discovered and reinvestigated Spumel- 

 larian families, Thalassothamnidae and Orosphaeridae, a totally 

 new type of nucleus has been found (Schroder and Hacker). It 



consists of a discoid structure 

 (*1 mm. diam.) enveloped by 

 a crenate membrane, and is- 

 composed of a thin cortical 

 substance and a central mass- 

 of very distinct nucleoplasm, 

 the cortical and medullary 



Cfi. 



FIG. 13. 



The central capsule and nucleus of Tusmrora , r ui 



(After Borgert.) As, the astropyle; gated, ICebly 



substances being separated 

 apparently by a membrane 

 (Fig. 14). The central 

 nucleoplasm contains segre- 



naiionalis. 



naonas. er orgert. As, the astropyle ; 



/'a, the two parapylae ; Nu, the nucleus with its o-rq^npq mrpr n an 



chromatin band (CVi). x 45. granil 



achromatic matrix ; the cor- 



tical layer, on the other hand, is densely chromatised. The 

 most striking feature of this nucleus is perhaps the presence of 

 lenticular bodies at intervals along the junction of its two com- 

 ponent layers, or in one genus (Orosphaera) just outside it. These 

 contain large compact lumps of chromatin imbedded in a less- 

 densely staining medium. In addition to this central nucleus, 

 scattered, chromatin-like granules (Fig. 14, s) occur in the endoplasm, 

 and in Orosphaera these peripheral granules are unmistakable 

 nuclei of a simple character. 



The shape and size of the nucleus often undergo considerable 

 change during growth. It remains vesicular, large, and spherical,. 



Fio. 14. 



Portion of a section through the 

 branched central capsule of Thalasso- 

 ttiamnus. (After Hacker.) The centre 

 of the capsule with its nucleus (N), endo- 

 plasni, and inclusions are shown. The 

 stratified concretions (s) stain with 

 haematoxylin, and are probably chro- 

 midial structures. In Orosphnera (a 

 genus which, according to Hiicker, is 

 closely allied to ThulassotMrnnua) these 

 peripheral nucleoplasmic structures are 

 capable of division. The nucleus (N) 

 shows well the division into crenate 

 membrane, peripheral chromatic layer, 

 and the central, mainly achromatic sub- 

 stance in which groups of staining 

 granules occur. Large lenticular bodies 

 (/) of unknown significance occur also. 



N 



and more or less chromatised in the Thalassicollidae and 

 Physematiidae ; but in the Thalassophysidae it becomes papillose, 

 elongate, and serpentiform, its plasma not only differentiates into 

 inner and outer substances, but the spherical or thread -like 



