THE CELL-THEORY 263 



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by extensive plane surfaces, whereas the even action of two attractive 

 centres, in a mass free to move, would give rise to two spheroids in r,, \ 

 contact only by a point. Again, Remak has observed that in the y 

 frog's egg the time occupied by the formation of the groove, indicating 

 the first line of cleavage upon the upper half of the yelk, is very much 

 shorter than that required to give rise to the corresponding line upon y, 

 the lower half — a fact which is quite unintelligible upon the theory of 

 a central attraction. 



Thirdly, in Cucullanus, Ascaris dentata, &c., Kolliker has shown 

 that, though nuclei are developed, no yelk-division occurs ; and in the 

 later stages of division of the frog's egg yelk masses are found undi- 

 vided, and containing many nuclei. 



Finally, in Ascaris mystax, according to Dr. Nelson,^ the embryonic 

 vesicles absolutely revolve in circles during the progress of yelk- 

 division — a phenomenon which seems incompatible with the existence 

 of any mutual attractive reaction between themselves and the vitelline 

 mass. 



We see, in short, that the effects of the force supposed to be exerted 

 by the nuclei may take place without them, and, on the other hand, 

 that the nuclei may be present without exerting the peculiar forces 

 which they are supposed to possess ; and finally that even if such 

 forces exist, they must be something very different from all the 

 attractive forces of which we have any conception ; and therefore that 

 the hypothesis of nuclear force is no explanation, but merely a fresh 

 name for the difficulty. 



We are as little able to discover any evidence of the existence of 

 metabolic forces in the nuclei. The metabolic changes of the tissues — 

 such as we see, for instance, in the conversion of cartilage into bone, 

 of cartilage into connective tissue — do not take place, either primarily 

 or with greater intensity, in the neighbourhood of the nuclei ; a fact 

 of which striking evidence is afforded by ossifying cartilage, in which 

 the first deposit of calcareous matter occurs, not in areae surrounding 

 each nucleus, as we should expect if they exerted a metabolic influence, 

 but in straight lines, v/hich stretch from the ossified surface into the 

 substance of the matrix of the cartilage, and the amount of calcareous 

 matter in which gradually diminishes as we recede from the ossified 

 part, without the least reference to the nuclei. It is the same with the 

 metamorphosis of the periplast of cartilage when it passes into tendon. 



From all this we consider it to be satisfactorily shown, that there is 

 no evidence that the " cells " of living bodies are, in any respect, centres 



Philosophical Transactions, 1852. 



