No. 578] SHORTER ARTICLES AND DISCUSSION 



12.) 



7me note. Les determinants de la couleur chez les souris, Stude 

 comparative. Ibid., Notes et revue (4), Vol. 8, pp. xl-lvi. 

 gan, T. H. 



908. Some Experiments in Heredity in Mice. Sci., N. S., Vol. 27, 

 p. 493. 



911. The Influence of Heredity and of Environment in Determining 

 the Coat Color in Mice. Annals N. Y. Acad, of Set, Vol. 21, 

 pp. 87-117. 



914. Multiple Allelomorphs in Mice. Am. Nat., Vol. 48, pp. 449-458. 



ON THE TIME OF SEGREGATION OF GENETIC 

 FACTORS IN PLANTS 



In (Enothera lamarckiana, Geerts (3) has observed that two mi- 

 crospores of each tetrad abort. From the results of reciprocal 

 crosses, De Vries (9) concluded that there was a segregation of 

 genetic factors between the aborted and unaborted pollen-grains. 

 In my crosses of StizoloUum species (2), half of the pollen-grains 

 abort in a random manner in the anthers of the F 1 hybrids ; and 

 I can only explain the results of the breeding work on the hypoth- 

 esis that there is a segregation between the four microspores of 

 each tetrad. Hence I conclude that the segregation does not take 

 place before the cell-divisions which form the pollen-mother-cells, 

 but takes place in the divisions which form the microspores. In 

 other words, segregation occurs here, not among the cells of the 

 diploid generation, but at the moment of formation of the indi- 

 viduals of the haploid generation. 



In the ovules of StizoloUum crosses I have shown that there 

 is a random segregation of aborted and normal embryo-sacs ; and 

 this agrees with the observations of Geerts on the functional mega- 

 spores of O. lamarckiana. If somatic segregation occurred, there 

 would be a segregation of whole ovaries or parts of ovaries with 

 ovules all aborted or all normal, causing a distribution which 

 would differ markedly from the binomial distribution demanded 

 by a random segregation according to the law of chance. I have 

 shown that with lots of n ovules each, the distribution of the 

 aborted and normal ovules corresponds to the binomial (1 + l) w . 

 Hence segregation can not have taken place before the formation 

 of the nucellus of the ovule. 



In many species and varieties of Citrus, as Strasburger (7) 



