THE RADIOLARIA 123 



chromatin accumulates at its periphery (Fig. 21, A, B). In the 

 Sphaeroidea the nucleus becomes tubercular and follows the growth 

 of the central capsule, as this encloses successive shells. In the 

 simpler forms of Nassellaria the vesicular nucleus remains elliptical, 

 but in the Cyrtoidea, in which it lies near the apex of the shell, it 

 sends lobes (Fig. 4, 3) into the adjoining lappets. 



The multinucleate Radiolaria offer other distinctive characters. 

 In the Sphaerozoa each individual of the colony possesses a gradu- 

 ally increasing number of structureless, singly refracting nuclei, 

 which multiply by direct fission, and have rather the appearance of 

 nuclear fragments than of true nuclei. In the case of Collosphaera 

 and Siphonosphaera, scattered chromidia (not associated as far as is 

 known with reproduction) occur as well. In Acantharia the multiple 

 nuclei have apparently a membrane and nucleoli, the multinucleate 

 condition is constant, and the distinction drawn by Haeckel between 

 such forms and oligo- or mononucleate Acantharia is a mistaken one 

 due to the presence of a parasitic Amoebophrya (Acinetaria), which 

 was mistaken for a nucleus (Part I. p. 423, Fig. 90). More difficult 

 to account for is the careful description by Hertwig of a temporary 

 nuclear condition discovered by him in a species of Acanthometron 

 and of Amphilonche. In the comparatively few nuclei of young 

 specimens, Hertwig found that the membrane became invaginated on 

 its peripheral side, whilst the massive nucleolus showed differentia- 

 tion into two parts. The neck of the infolded membrane became 

 radiately arranged, and its deeper portion creased into circular 

 folds lying one over another. After a time these appearances 

 vanished and the nuclei resumed their simple spherical form. The 

 phenomenon may be one of internal budding. 



The advent of sporulation is prefaced and accompanied by 

 changes in the nucleus. These changes, however, are but imper- 

 fectly known (p. 139). Vesicular nuclei shrink, their membrane gives 

 way, and the altered chromatin and enclosed nucleoplasm either 

 flows out into the endoplasm or gives rise to a nuclear figure and then 

 disperses (see above, pp. 99-100, for Thalassicolla). From the frag- 

 mented material spore-nuclei arise. By the former method isospore-, 

 by the latter heterospore-, nuclei develop. In the Sphaerozoa, how- 

 ever, the homogeneous scattered nuclei remain undifferentiated 

 during the formation of isospores, and only exhibit a change from a 

 singly to a doubly refractive property ; but previous to the develop- 

 ment of heterospores their nuclei become modified into chromatic 

 and achromatic portions, which are further differentiated in the 

 mega- and microspores. 



In the Phaeodaria the ellipsoidal nucleus is usually a single 

 large structure, but two or three nuclei may be present. It con- 

 sists (Fig. 14) of a membrane containing a linin network. The 

 chromatin is massed at the centre, and from this point radiating 



