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from chemical considerations that if the lecithin is split up, and 

 the phosphorus abstracted from it, a substance known as cholin 

 must be set free. 



Let us turn for a moment to another and apparently discon- 

 nected series of facts. If we take a drop of any oil, such as olive 

 oil, and float it on the surface of pure water, the drop of oil, of 

 course, does not mix with the water, but simply floats on the top. 

 If we take a very fine cotton thread and soak it in cholin, which 

 is a liquid, and then just touch the surface of this droplet with 

 the thread, the droplet immediately divides into two. Under- 

 stand, we are not cutting the droplet with this thread. If we 

 tried to cut the oil with a thread, dry or wet with water, we 

 should fail, but if we wet the thread with cholin and apply it to 

 the oil the latter immediately splits into two, and we get two 

 separate droplets floating on the water. The physical mechanism 

 of this is perfectly well understood, but it involves certain rather 

 abstruse principles upon which I shall not dwell to-night. Now, 

 you recollect I told you that in the production of the nuclear 

 material the only possible source of the phosphorus must be the 

 lecithin existing* within the cell, and that, in taking phosphorus 

 from the lecithin, the nucleus must leave behind a quantity of 

 cholin. Let us suppose that the nucleus in this original cell is 

 divided into two nuclei which migrate to the two opposite poles 

 of the cell, and proceed to grow. They can only grow by taking 

 the phosphorus away from the lecithin, and as they do that cholin 

 must be set free. This cholin will immediately proceed to diffuse 

 through the body of the cell as all substances in solution do. It 

 will diffuse in all directions equally, but you will observe that it 

 will diffuse towards the middle of the cell from two sources, 

 whereas towards the outer edge of the cell it will diffuse from only 

 one source, so that the middle of the cell is receiving two supplies 

 of cholin, but on the outside it is receiving only one. The result 

 is that the concentration of cholin within the cell must be greatest 

 along the line lying equidistant from the two nuclei, and the cell 

 divides into two just as the oil droplet divides into two, and for 

 the same reason. We therefore understand the mechanism of this 

 rapid division into two more cells which occurs within this 

 original cell, and we can imitate it artificially with such things as 

 oil droplets at our will. 



It has become possible during recent years to control these 

 processes of cell division almost absolutely at our pleasure. Let us 

 take the case of this original cell which has divided into two. If 

 we wish, we can make those two cells melt into one again by the 

 very simple method of adding to the sea water a little alkali, 

 something of the nature of soda — washing soda. These two cells 

 fuse into one when treated in that manner. What happens then? 

 We get a giant embryo — double the normal size. We can do more 

 than that. We can cause those two cells to fall apart entirely, 

 and no longer adhere along their line of division. What happens? 

 We obtain twins — two embryos, one developed from each cell. 

 The method by which we can do this is very simple. Excluding a 

 few substances existing in minute quantities, ordinary sea water 

 consists practically of certain salts dissolved in water in the 

 following proportions: — 1,000 parts of chloride of sodium, which 

 is ordinary table salt ; 78 parts of chloride of magnesium ; 38 parts 

 of sulphate of magnesium ; 22 parts of chloride of potash ; and 10 

 parts of chloride of lime. It is easily proved by experiment that, 

 for the normal fertilization and development of sea-urchin eggs. 



