ioo THE RADJOLAR1A 



almost homogeneous and doubly refractive character, and becomes 

 irregular in shape as its membrane disappears. By what appears 

 to be amitotic division the nucleus fragments into a large number 

 of equivalent pieces, each of which behaves as an independent 

 nucleus, and by further division these nuclei become disseminated 

 through the endoplasm. Around each nucleus the cytoplasm con- 

 denses to form an ovoid mass, which is differentiated at the nuclear 

 pole into two cilia. Meantime the reserve materials of the endo- 

 plasm become subdivided and apportioned, so that each isospore 

 contains a few granules of fat and a crystalloid. These changes 

 may be followed on the accompanying figures (Fig. 2, A-D). 



The development of heterospores in Thalassicolla proceeds in a 

 different manner and from distinct individuals. The first step is 

 the formation of a nuclear figure. A clear achromatic vesicle 

 (centrosome, Brandt, 1905) arises in the nucleus and becomes 

 surrounded by granular radiations, upon which the thick bent 

 chromatin threads arrange themselves as in Fig. 2, E. The centro- 

 some now shifts towards the margin of the nucleus, and the more 

 peripheral chromosomes become lumpy and slightly vacuolated. 

 The nuclear wall softens, and through it, at one pole, pass the 

 centrosome and a few apical chromatin granules. Subsequently the 

 nuclear sap escapes over the entire periphery of the nucleus, 

 together with much of the granular nuclear matrix, into the sur- 

 rounding endoplasm. The chromatin threads fragment and the 

 fragments become associated with segregated masses of fine nuclear 

 granules to form organised nuclei, which divide mitotically. During 

 this process the nuclei are carried outwards in increasing numbers 

 towards the wall of the central capsule, where they become 

 arranged in columns, until almost the whole of the original nucleo- 

 plasm is used up. The most remarkable features of this organising 

 process is that the developing nuclei are of two sizes, which are 

 severally aggregated in the peripheral columns. Meantime the 

 endoplasm and its reserves have been mobilised. The former is 

 converted into cylinders around the mega- or micro-nuclei, and 

 within these cylinders the fat and crystalloids become fragmented 

 and distributed. Finally, by subdivision of these nucleated masses 

 colonies of mega- and micro-spores arise. Both are biciliate, and in 

 comparison with isospores minute, and divided by a groove into a 

 reniform shape. The microspores are from O'OOS to O'Ol mm. in 

 length, the megaspores 0'016 to O'OIT. The microspores have a 

 deeply staining granular nucleus and a cytoplasm free from inclu- 

 sions except for one or two minute crystalloids. The megaspores, 

 on the other hand, possess a nucleus poor in chromatin, and their 

 cytoplasm is crowded with refringent corpuscles. Both forms of 

 heterospore have the same ciliary mechanism (Fig. 2, L). From 

 one point in the groove two long cilia arise, one of which works 



