AND LIFE-HISTORY OF DIATOMS. 425 



particular case, I shall at the same time be broadly summarising 

 the conditions existing in the other Diatoms of which he treats, 

 such as Nitzschia sigmoidea, Phinularia major, viridis, and 

 ohlonga, Pleurosigma attemcatum, and JVavicula amjjhishaena. 



The first sign that Surirella is about to undergo division is 

 afforded by the frustule, the valves of which move apart from 

 one another to a perceptible extent, so that the whole Diatom 

 becomes obviously broader. At the same time the chromatophores 

 begin to lose their normal contour little by little, and the 

 processes which usually occupy the transverse canals in each 

 valve are gradually retracted, so that the latter are eventually 

 left quite unoccupied. The centrosome, which in the resting 

 cell lies in the hollow of the kidney-shaped nucleus, gradually 

 moves outwards and becomes the centre of a system of 

 radiating lines traversing the cytoplasm in all directions, as 

 mentioned earlier in this paper. There is as yet no perceptible 

 alteration in the nucleus, which exhibits the normal arrangement 

 of a network of fibrils composed of a substance called linin, in 

 the interstices of which are scattered numerous gi^anules of the 

 deeply staining substance called chromatin, or nuclein. 



It is at this stage that a small, pale, spherical body makes its 

 appearance in the immediate vicinity of the centrosome, from 

 which, according to Lauterborn, it is without doubt derived by a 

 process akin to budding. This insignificant particle is of great 

 interest, representing as it does a structure as yet undeveloped, 

 but destined, as the so-called "central spindle," to play a most 

 important part in the division of the nucleus. 



A. little later and we see the nucleus involved in the on- 

 coming change. Its reticulum begins, if one may use such an 

 expression, to shake itself out, so that the whole texture becomes 

 looser, but the most marked feature is the commencing arrange- 

 ment of the chromatin granules to form single strands looking 

 like strings of beads, a condition first observable at the periphery of 

 the nucleus, from whence it spreads inwards towards the centre. 

 The rudimentary central spindle has also moved away from the 

 centrosome towards the surface of the nucleus, and has increased 

 markedly in size. In fact, at this stage it closely resembles one 



