142 



THE RADIOLARIA 



conical structure ('012 mm. long). From its pointed end spring 

 the two cilia, one of which is usually held in a somewhat horizontal 

 position, the other curving backwards and downwards. Near this 

 end lies the nucleus, which has acquired, according to Brandt 

 (10, p. 163), a certain differentiation. The broader end is filled 

 with the crystal and granules (Fig. 26, E). 



Heterospwes, The formation of megaspores and microspores 

 may proceed from the same (Sphaerozoidae) or separate colonies 



FIG. 26. 



Isospores and heterospores of Radiolaria. A, heterospores of Xiphacantha alata (Acantharia). 

 B, isospores of the same. C and D, microspore and megaspore of Collozoum inerme. E, 

 isospore of the same showing crystal and inclusions. F and G, megaspores of Sphaerozoum sp. 

 H, microspores of the same. (After Brandt.) 



(Collosphaeridae). The process differs from the development of 

 isospores in the presence of segregated nuclei, the differentiation in 

 the nuclei of achromatic substance, and the dimorphism of the mega- 

 and micro-nuclei. In the Collosphaeridae the full-grown vegetative 

 .colony shows the first traces of heterospore- formation by the 

 segregation of its homogeneous nuclei into groups of 2, 4, or 8. 

 This arrangement is temporary, and very soon the nuclei are 

 found arranged in several layers, each nucleus being now clearly 

 composed of a highly refractive and achromatic ground-sub- 

 >stance, in which are imbedded thread-like masses of chromatin. 



