270 Moment of Segregation [ch. 



begins to acquire its own special features, and we may 

 without much difficulty imagine that if two daughter-cells 

 are to be differentiated allelomorphically from each other, 

 the differentiation will come about at this reduction, or 

 meiotic division, as it is called. When however we seek for 

 proof, there is as yet none which is quite convincing. The 

 one observation bearing immediately on the problem is 

 that to which Correns has appealed '^. In the maize we 

 know that those F^ seeds which have wTinkled or sugary 

 endosperms will give rise to plants with similar sugary 

 seeds. But the seed of maize is formed by a double fertili- 

 sation. It consists of two parts, an embryo, and an endo- 

 sperm, and it is the sugary condition of the reserve-material 

 in the endosperm which causes the wrinkled appearance of 

 the seed. From the fact that wrinkled seeds always give 

 rise to plants bearing wrinkled seeds, it is clear that the 

 same elements have entered into the composition of both 

 the embryo and the endosperm of the same seed. But the 

 embryo is formed by the union of one nucleus of the egg- 

 cell with one from the pollen-tube, and the endosperm is 

 similarly , formed by the union of the united polar nuclei 

 with another from the pollen-tube. Hence we may infer 

 that the two nuclei brought in from the pollen-grain bear 

 similar allelomorphs, and that all the nuclei of the egg- 

 cell must also be similar in composition. Therefore the 

 segregation of characters cannot in the maize take place 

 after the division by which the pollen-grain was formed, 

 for both nuclei to which it gives rise are of the same 

 composition ; and the sam e argum ent applies to the egg-cell. 



Chromosomes as the possible Bearers of Factors. ^ 



These are, I believe, at present, the only facts which 

 positively limit the inquiry. It has been pointed out by the 

 cytologists that the details of the processes by which nuclear 

 reduction is accomplished may be readily construed as 

 effecting the operation of segregation, but while admitting 

 this as a fair and even probable interpretation, nothing 

 in my judgment yet compels us to accept it as proved. 



* The same conclusion is strongly supported by the evidence frona 

 Hieraciu7n (see p. 247). 



