328 NATURAL SCIENCE. November. 



the forces which are operating in the cell, and which finally produce 

 the phenomena of karyokinesis ; and the extreme advocates of these 

 respective views only weaken their case, as well as cause a difficult 

 subject to become still more obscure, by refusing to look at the other 

 side of the question. For, while it is certain that in some cells the 

 centrosome does possess a marked individuality of existence, it must 

 be remembered that this fact is not one of general application ; hence 

 it is absurd to speak of it as a " necessary organ " of division. The 

 recent studies of Hertwig, for example, on the behaviour of un- 

 fertilised echinoderm eggs, as well as the observations of botanists, 

 have shown that nuclear division may be initiated and even 

 completely gone through without the occurrence of a visible 

 centrosome ; and further, in certain other cases {e.g., spore-mother- 

 cells of liverworts), where these bodies are present there is absolutely 

 no proof that they are anything but new and temporary formations in 



the cell. 



In fact, there is but shadowy evidence, even in the most 

 favourable instances, that the centrosomes act as principals in the 

 process of karyokinesis ; they may well only be aggregations which 

 mark the centres of the forces operating within the protoplasm. 

 They would occupy a similar position whether they were the active 

 agents, or merely the passive insertion points of the radiations. We 

 know that contractihty is a property of protoplasm, and there is 

 no a priori need for assuming a special organ to direct the procedure 

 of that special kind of contractility which results in the partition of 

 the nucleus. Post hoc does not always mean propter hoc. 



But these considerations do not affect the further conclusion, that 

 a certain structure may be produced by a definite interaction of forces 

 upon matter, and that in the case of protoplasm this might lead to an 

 aggregation of such a character as that to which we give the name of 

 centrosome ; and that, further, the same set of physical and 

 mechanical conditions being periodically reproduced, would lead to 

 as regular a reappearance of the structure — the centrosome. From 

 what we know of protoplasm it would not be difficult to imagine that 

 something like this does go on in cells, and possibly this may account 

 for the periodical reappearance, and even permanence, of these bodies 

 in some of the cases in which they have been observed. Still, 

 it must be remembered that we actually know so little about 

 protoplasm • that any such line of thought is merely speculative, 

 although it may serve for the present to link together the apparently 

 irreconcilable results which have been reached by different in- 

 vestigators. 



The present position of the question as to the origin and nature of 

 the achromatic spindle is also in a very unsatisfactory state. Does 

 the spindle arise as the result of an onward development of a 

 pre-existing rudiment, or is it a new formation in the protoplasm ? 

 In the answer to this question, no less than in the conclusion to which 



