214 THE CELL 



certain general relationships which exist between the nucleus 

 and the protoplasm, and to which I have drawn attention in my 

 paper upon the influence exerted by gravitation upon cell 

 division (VI. 31). 



In the resting cell the nucleus may occupy various positions ; 

 it may also change its place, as, for instance, in plant cells, where 

 it may be carried along by the protoplasmic stream. However, 

 under certain conditions, of .which only those connected with cell 

 division will be entered into here, whilst others will be mentioned 

 later on in Chapter VIIL, the nucleus occupies a definite constant 

 position in relation to the protoplasmic body. 



Certain interactions take place between the protoplasm and the 

 nucleus during division, similar to those which (to use a familiar 

 illustration) exist between iron filings and a magnet suspended 

 loosely over them. The magnetic influence polarises the iron 

 filings, causing them to group themselves radially about the poles. 

 On the other hand, the whole mass of the polarised particles of 

 iron has a directing influence upon the position of the magnet. 

 These metastatic reactions between protoplasm and nucleus re- 

 ceive their evident expression in the appearance of the pole centres 

 and the radiation figures, which have been already described. 

 The result of the reaction is that the nucleus always endeavours 

 to occupy the centre of the reaction sphere. 



No objects are more suitable for demonstrating this than animal 

 ova, which may vary considerably as regards size, shape, and in- 

 ternal organisation. 



In most small ova, in which protoplasm and yolk substance are 

 more or less evenly distributed, the nucleus, before fertilisation 

 (Fig. 106.4), does not occupy any definite position. On the other 

 hand, when, after fertilisation, it commences to be active and to 

 divide (Fig. 106 B), it places itself exactly in the geometrical 

 median point, that is to say, if the egg is spherical in the centre, 

 or if it is oval (Fig. 110) in the point of intersection of the two 

 longitudinal axes. The nucleus surrounded by a radiation sphere 

 may be seen to travel through the protoplasm to this point. 



Variations from the normal are seen when the protoplasm and 

 yolk granules, of which the latter, as a rule, have the greater 

 specific gravity, are unevenly distributed in the egg cavity. Very 

 frequently the eggs undergo a polar differentiation, which is partly 

 produced directly by gravity, the various substances being sepa- 

 rated out according to the weights, and partly by other processes 



