i6o Discussion of Coupling [ch. 



three times, the series ZAB + \Ab + \aB + ^ab would 

 result. We cannot, on .the observed numbers, assert 

 quite positively that the system is not 8 : i, but as obser- 

 vations accumulate, this supposition becomes increasingly 

 improbable, for the numbers all point rather to 7 : i 

 than 8:1. 



If, as many suppose, the whole process of segregation 

 is completed in the reduction-division, it is obvious that 

 any suggestion involving successive segregations fails. Still 

 it is worth noting that nothing yet limits us to the con- 

 ception of segregation as occurring all at once. We know 

 very little yet as to the cytological processes antecedent to 

 the reduction-division. Moreover it cannot yet be asserted 

 that all the gametes, or even all the gametes of one sex 

 (in hermaphrodite forms) are in the same cell-generation, 

 counting from the first cleavage-plane of the zygote. 



It is to be noted also that where the germ-cells are 

 many, as in the testes of animals and the anthers of most 

 plants, it is not difficult to imagine the formation of even very 

 long series of couplings. The egg-cells, on the contrary, 

 are few, and in plants they are very often definitely grouped 

 in special organs which again are arranged on a definite 

 geometrical plan relatively to the gross anatomy of the 

 plant. Even if the various accessory cells of the plant 

 ovary are reckoned as belonging to the gametic series, the 

 number still seems insufficient to allow for the development 

 of a coupling which demands a long series for its expression. 

 The question may naturally be asked whether there is any 

 organised system of differentiation connecting the several 

 ovaries into a common plan. The differentiation among the 

 egg-cells might conceivably be distributed on a geometrical 

 plan like the differentiation among the somatic organs of the 

 plant. All the available evidence is however against this 

 suggestion, for in maize and peas, where indications of this 

 system might be found if they existed, all the evidence 

 is entirely negative. 



There is still another direction in which we may look for 

 an elucidation of the nature of gametic coupling. If the 

 factors can act upon each other in such a way that certain 

 combinations do not occur, as we have already seen actually 

 happening in the case of Spurious Allelomorphism, it seems 



