MORBID GKOWTH IN GENERAL. 73 



^ 6S. The second stage in cell-development is fission of the 

 CELL itself. Each of the newly-formed nuclei operates as a centre 

 of attraction for the protoplasm which immediately surrounds it ; 



Fig. -27. 









\ 



Giant-cells, a. 'Rounded {Virclioiv) ; Z^. With processes : from a 

 muscular tumour {BiUrotJi). 



and when this attraction goes so far as to isolate a segment of 

 protoplasm round each nucleus, we say that the cell has under- 

 gone division. 



The microscopic appearances presented by this ^' segmenta- 

 tion " of the protoplasm are not alike in all cases. The leading dif- 

 ferences are due to the degree of cohesion to which the peripheric 

 layer of the j3arent-cell has attained. This outermost layer is 

 in most cells of extreme tenuity — a " physical membrancj" such 

 as is always formed at the junction of two dissimilar fluids which 

 refuse to mingle with each other. The greater the size and the 

 age of the cells, the more distinctly are they seen to be invested 

 by a colourless, homogeneous, highly refractive, double-contoured 

 membrane — a cell-membrane in the old sense of the word.* 



* M. Traiibe tries to find a chemical explanation of the phenomenon 

 in the precipitation of an albuminous constituent of the protoplasm by 

 another colloid {Gmliam) substance acting upon the cell from without. 

 Kilhne^s experiments on Infusoria (Amoeboe) agree very imperfectly 

 with this hypothesis. It is very certain that a membrane appears on 

 the surface of the amoeba in consequence of external irritation; but 

 Kllhne finds that the membrane-producing irritants are rather of a 

 physical than a chemical nature (electricity, alterations in temperature) ; 

 hence Killine contents himself with defining the membrane-formation 

 simply as a peripheral coagulation of the protoplasm. 



