THE CELL AND ITS CONSTITUENT STRUCTURES. 147 



clear during the actual stages of nuclear division. Whether 

 they retain their identity during the actually resting condi- 

 tion seems to me to be doubtful. At any rate, it would 

 require some confidence to discriminate them in the absence 

 of all radiations, from the other granules which lie in the 

 vicinity of the nucleus, each likewise surrounded by a clear 

 sphere. In Liverworts again they are often extremely well 

 defined, and at the same time they appear in such a peculiar 

 fashion in many of these plants that I may perhaps be par- 

 doned for drawing special attention to them here. It has 

 not, so far, been found possible to detect with certainty the 

 existence of centrosomes in the resting cells of these plants, 

 but when nuclear division is about to take place, a minute 

 but easily recognisable body is found at the spot from which 

 the radiations are beginning to appear. During the suc- 

 cessive bipartltions which go on in the young sporogenous 

 tissue of such a liverwort as e.o\ Fossombronia^ they are 

 particularly plain at the earliest stages of karyokinesis. But 

 when the spindle is complete and the chromosomes are 

 forming the equatorial plate, the radiations die away at the 

 poles and then a centrosome is no longer distinguishable. 

 The same thing may be still more easily seen during the 

 germination of the spores of Pellia, a very common 

 Hepatic ; and convenient, because the spores germinate 

 while still within the sporogonium, and hence all stages 

 may be found close together. But as the process of karyo- 

 kinesis advances the radiations once more appear, and 

 with them, the centrosphere usually again becomes visible. 

 Is it possible that this temporary obliteration of the polar 

 radiations is to be taken as the expression of a relatively 

 stable, if transient, condition of equilibrium, and that, with 

 the temporary cessation of movement in the protoplasm, the 

 differentiation of the centrosphere and the attendant radia- 

 tions are for the time being in abeyance ? 



Perhaps some one may object that, after all, I have as 

 yet omitted to state the strongest evidence for the claims of 



^ Farmer, " Nuclear Division and Spore Formation in the Hepaticae,"' 

 Annals of Botany, vol. ix., 1895. 



