FOUR IMPORTANT THEOREMS 51 



but never fused, to form one nucleus — the segmentation-nucleus 

 — the cleavage or segmentation of the fertilised ovum begins. 



There is a centrosome, derived from the sperm-centrosome, 

 at each pole of the nucleus, and a system of fine rays radiates 

 from each, some of these rays entering into close association with 

 the chromosomes. 



Each chromosome is halved longitudinally, as a piece of stick 

 might be split up the middle, and after a very complex routine 

 the halves of each split chromosome migrate, either actively or 

 passively, to opposite poles. Thus, near each centrosome there 

 comes to be a group of chromosomes, half of each group being 

 of paternal origin and half of maternal origin. Each group in 

 an orderly fashion rounds itself off into a unified nucleus, the body 

 of the cell (the cytoplasm) constricts across the equatorial plane, 

 and two cells are formed. 



The gist and import of the whole process is the precisely equal 

 partition of the maternal and paternal contributions, so that 

 each of the daughter-cells has a nucleus half maternal and half 

 paternal. For many successive divisions (e.g. in Cyclops) the 

 duality has been demonstrated,* so that we may fairly say that 

 the maternal and paternal contributions form the warp and 

 woof of the growing orgaaism. 



2. Inheritance, though Dual, is strictly Multiple. — 

 Although the whole inheritance which constitutes an offspring 

 usually comes from two parents, and may therefore be called 

 dual, it is obvious that the heritable material of each parent 

 was also dual, being derived from the grandparents, and so on 

 backwards ; so that inheritance is strictly not merely dual, but 

 in an even deeper sense multiple. Amphimixis or fertilisation 

 implies the subtle mingling of two minute organisations so that 

 they become physiologically one, but each of them was already 



* According to Haecker's careful observations on the water-flea Cyclops, 

 the paternal and maternal contributions, i.e. chromosomes, are traceable 

 as distinct individualised items throughout the whole of development. 



