331 



which for various reasons seems improbable. There is therefore 

 much reason for thinking that segregation can occur before 

 gameto-genesis begins, but there is no indicatioü as to which 

 are the critical divisions. 



Now that we may regard the formation of four cells of 

 composition AB, Ab ; aB ; ab, as the foundation both of the 

 coupling — and of the repulsion — series the problem in mani- 

 festly somewhat simplified. The time, excluding gameto-genesis, 

 at which we can most readily imagine four such delinite quadrants 

 to be formed is during the delimitation of the embryonic tissues. 

 It is then that the plant is most clearly a single geometrical 

 System. Moreover the excess of gametes jof parental composition 

 characterizing the coupling — and repulsion — series must certainly 

 mean that the position of the planes of division by which the four 

 quadrants are constituted is determined with regard to the 

 gametes taking part in fertilisation. Though the relative positions 

 of the constituents of the cells may perhaps be maintained 

 throughout the history ot the tissues, it is easier to suppose that 

 the original planes of embryonic division are determined according 

 to those positions than that their influence can operate after 

 complex somatic differentiation has been brought about. 



At some early stage in the embryonic development or per- 

 haps in later apical divisions we can suppose that the n— 1 cells 

 of the parental Constitution are formed by successive periclinal 

 and anticlinal divisions of the original quadrants which occupy 

 corresponding positions. The accompanying diagram gives a 

 schematic representation of the process as we imagine it. (Fig. 3.) 



Whatever hypothesis be assumed the following points remain 

 for consideration. 



1. We are as yet unable to imagine any simple System by 

 which the four original quadrants can be formed by two s i m i 1 a r 

 divisions. Evidently there must be two cell-divisions ; and if in 

 one of them we suppose AB to separate from ab, we cannot 

 then represent the formation of Ab and aB. 



Therefore we are almost compelled to suppose that the 

 original zygotic cell forms two similar halves, each AaBb. and 

 that the next division passes differently throngh each of these 

 two halves, in the one half separating AB from ab, and in the 

 other half separating Ab from aB. The axes of this System may 

 well be determined by the position of the constituent parental 



