124 THE RADIOLARIA 



strands, threads, and lumps run outwards towards the periphery. 

 In addition to these chromatised elements, threads and granules of 

 another substance, the so-called paranuclein of Borgert (18), are 

 present. Nucleoli are absent. 



The phenomena of nuclear division in this group have been 

 carefully studied by Borgert (17, 18) and Karawiew (16) in 

 Aulacantha scolymantha, but only mitotic division has been fully 

 described. Direct division of the nucleus without elongation is 

 known, but only a preliminary account has as yet been published. 

 The behaviour of the nucleus during sporulation is unknown. 

 Nuclear mitosis in Aulacantha exhibits five phases. In the first 

 or spirem stage, the linin threads form a dense coil, along which 

 the chromatin becomes arranged in a moniliform fashion ; a few 

 remnants, together with the paranuclein, lie scattered through the 

 nucleoplasm. The coil is in all probability never a continuous 

 thread, and no distinct centrosomes appear at this or any subse- 

 quent phase. The next stage consists of two events. The threads 

 of chromatin become cut up into varying lengths, and split longi- 

 tudinally so as to form rows of chromatin globules on either 

 side of the linin threads. The second event is the condensation of 

 these globules into thick short lengths of double chromosomes. 

 The pairs so formed are unequal in size and different in form, 

 some being spherical, some elongated or rod-like, but the members 

 of a pair are alike. Amongst these the paranuclein granules lie in 

 isolated heaps. The next or third stage is characterised by a 

 second longitudinal splitting of the chromosomes in a plane at 

 right angles to the first. The fission products separate, elongate, 

 and become thinner and twisted, leading up to the fourth stage or 

 second spirem phase, which is so far different from the first in that 

 the chromatin elements are obviously discontinuous, and the nucleus 

 as a whole has now become flattened, discoidal, and bent, in conse- 

 quence of the loss of its membrane, so that it presents, in side 

 view, a somewhat triangular outline, the apex directed towards the 

 astropyle. The large mass of chromosomes is now organised on 

 either side of a median transverse plane passing at right angles to 

 the flattened nucleus. The position of this plane is occupied by a 

 mass of short chromatin elements and debris, between which para- 

 nuclein granules occur. The chromosomes are aggregated on each 

 side of this central mass, which prevents them from being continuous 

 from one side of the nucleus to the other, and are more densely 

 crowded near the centre. The whole flattened structure stretches 

 out until on the aboral side it touches the central capsule. The 

 fifth phase is signalised by the appearance of the equatorial plate. 

 The origin of this structure has not been described, but the 

 chromosomes now arrange themselves in close relation to it, and 

 become heaped up in parallel series, though still maintaining marked 



