484 DEVELOPMENT 



where an oil drop is divided by the imposition on it of a thread 

 soaked in alkah, are not very ihuminating. The energy of cell 

 division is not external to the cell, but depends entirely on a 

 redistribution of forces inside the cell. 



We have seen that, as a result of the diminution of specific 

 surface with growth, metabolic processes are retarded and acid 

 by-products tend to accumulate. It is obvious that at the centre 

 of the cell, i.e. at the region most distant from the surface, these 

 changes due to maloxidation will be most marked. This decrease 

 in metabolism is accentuated by the fact that the nucleus, which 

 is always in the centre of the field of energy of the cell and usually 

 near the centre of the cell material, is the seat of the most rapid 

 oxidation changes in the cell. It will, therefore, show the effects 

 of the lack of oxygen at a very early stage. It follows from this, 

 that the intrinsic energy of the fluid at the centre will either suffer 

 an increase or a decrease. Everything points to the latter as 

 occurring. Now, as surface tension is, as far as cell problems are 

 concerned, a relative magnitude, we may say that the tension 

 of the superficial layer of the cell which, on account of its proximity 

 to the surface, remains practically normal, increases in close corre- 

 spondence with the decrease of intrinsic energy of the central 

 portion. 



This increase is still further accentuated by the increase in the 

 relative " permeability " to energy of the surface of the cell, 

 which, as we have seen, causes the development ot radial lines of 

 force. The high tension of the surface of the Gg,g operating on the 

 central region of low intrinsic energy causes the material in the 

 centre to be dispersed into the cytoplasm of the cell. These 

 dispersed particles first set themselves in a neutral position. 

 i.e. on the plane of segmentation — cf. iron fdings in a magnetic 

 field of force between two similar poles. The position of the nucleus 

 determines the first plane of segmentation, since apparentl\ 

 nuclear division precedes the visible division of the cytoplasm of 

 the egg. In other words, the plane of nuclear division becomes 

 the plane of segmentation of the whole cell. The nuclear matter, 

 as manifested by the chromosomes, gradually sets in the lines of 

 force between the ccntrosomes and slowly separates into two equal 

 portions which ultimately form the two nuclei. Hence we have 

 a spherical cell which is capable of division into two exactly 

 similar portions. It is, therefore, manifest that each potential 

 segment, because of this similarity of structure and energy content, 

 will repel the other half and, according to the ordinary laws of 

 stresses and strains, will cause division in the plane of symmetry. 

 The high tension of the cell surfaces will ensure the continuity 



