FORMATION OF MONSTROSITIES 469 



or of a soap of choline or some other nitrogenous base, formed by com- 

 bination with the fatty-acid radicals of the phospholipin. Immediately 

 following the division of the cell-nucleus into two, which precedes by 

 a definite interval the division of the cell, we may suppose an active 

 synthesis of nuclear materials to be occurring in the two nuclear regions. 

 Hence, in these localities, provided that the above hypothesis be correct 

 choline or some other nitrogenous base would be set free. If now, choline 

 be liberated at both nuclei and diffuses from each nucleus equally in 

 all directions its maximal concentration must obviously occur in the 

 equatorial plane at right angles to the line joining the two nuclei. 

 We have seen that choline, when applied to the diameter of a droplet of 

 liquid immiscible with water (provided soap is formed) results in the 

 division of 'the drop along that diameter. It is possible that choline, 

 set free in nuclein synthesis, brings about, in a similar manner, the 

 division of the cell, through the formation of soaps in the equatorial 

 plane, either through combination with fatty acids in the cytoplasm, or 

 else through its having been liberated in the neighborhood of the nuclei 

 in combination with one or more of the oleic, stearic or palmitic acid 

 groups of the lecithin molecule. 



It is not even necessary to presuppose an actual separation of the 

 two nuclei; it is only necessary to suppose that the nuclein synthesis 

 occurs with greater rapidity at opposite poles than elsewhere within 

 the nucleus in order to understand how nuclear division may be brought 

 about by essentially the same mechanism as that which brings about 

 cell-division itself. 



ARTIFICIAL TWIN-FORMATION AND THE FORMATION OF 

 MONSTROSITIES. 



In the normal development of the egg the early cleavage-cells, 

 although distinct and separated from one another by a definite inter- 

 face, nevertheless remain in close apposition to one another, So long 

 as this is the case a single embryo develops. If, however, the first two 

 cleavage-cells chance to fall apart and cease to remain in their normal 

 closeness of apposition then each of the cells develops into a separate 

 and complete embryo and twins are formed from a single egg; these are 

 probably similar in origin to the "identical twins" which are occasion- 

 ally encountered among higher animals and man. 



It has been found by Loeb that the separation of the first cleavage- 

 cells may be brought about in over ninety per cent, of fertilized sea- 

 urchin eggs, provided they are merely exposed, for some time after 

 the first cell-division, to an artificial sea-water differing from normal 

 sea-water in the lack of any one of the constituents Sodium, Potassium 

 or Calcium. This change in the composition of the surrounding saline 

 medium apparently so alters the consistency of the surfaces of the 

 cleavage-cells that they no longer adhere to one another. It may be 

 noted that as the fertilization membrane still surrounds both of the 

 cleavage-cells and the composition of the external saline mixture can 

 nevertheless affect the surfaces of the eggs, the fertilization membrane 



