120 COMPARATIVE ANATOMY 



along the achromatic network, but much of it is concentrated 

 in the karyosome, where it forms a rind or investment to a 

 central corpuscle of achromatic plastin. The ribbon-like 

 spireme that emerges from the network exhibits a distinct 

 polarity, the loops of the ribbon being gathered up towards 

 the pole of the nucleus nearest to the centrosphere. The 

 karyosome occupies a more or less central position, with the 

 loops converging towards it. The spireme ribbon is at first 

 slender and the chromatin is evenly spaced along it in the 

 form of a single row of bead-like granules (fig. 25, B). After 

 a while the loops spread out again, the ribbon becomes thicker, 

 and there is an obvious increase of chromatic material. It is 

 probable that this increase is due in part at least to the 

 continual transference of chromatin from the karyosome 

 to the spireme. At all events the karyosome gradually 

 loses its chromatic investment and disintegrates, its remnants 

 being eventually cast out into the cytoplasm. As the ribbon 

 thickens it is split longitudinally throughout its entire length. 

 Each chromatin bead first divides into two and afterwards the 

 achromatic substance is divided, the spireme at this stage ap- 

 pearing as a double row of deeply stained beads supported by 

 two parallel ribbons of achromatic material (fig. 25, C). At the 

 same time the loops become more distinctly separated from 

 one another and contract towards the centrosphere. As con- 

 traction proceeds, the longitudinally split ribbon is divided 

 transversely into twelve U-shaped loops whose open ends are 

 directed towards the centrosphere. Close inspection shows 

 that each loop is formed of two bent segments united end to 

 end at the bend of the U, and this observation is confirmed by 

 the study of the subsequent history of the loops. Thus, though 

 there are apparently twelve U-shaped chromosomes that 

 is to say, half the number typical of the pre-meiotic divisions 

 there are in reality twenty-four, each loop being a double or, as 

 it is called, a bivalent chromosome, formed by the union of two 

 chromosomes end to end. The process which results in the 

 emergence of the chromosomes in pairs is known as synapsis. 

 The longitudinal split observed in the previous stage is also 

 recognisable in each bivalent chromosome, and in the accom- 

 panying figures it is represented as persisting through the meta- 

 phase and anaphase, though it usually disappears to reappear 

 again at a later period. It should be clearly understood that 



