477 



their division the singie chromosomes appear to arrange themselves 

 in the equatorial region much more uniformally than in a cor- 

 responding stage in P. INI. C. 



Tlie chalazal daugiiter nucleus is ahvays the more active and 

 passes into the second division in advance of the miciopylar, as 

 shown in fig. 2 c. In this division there is, apparently, much less 

 tendcncy to irrcgularity than in a similar stage in P. M. C. As 

 to the condilion of the chromosomes remaining and dividing in 

 the spindie in anaphase of the second division of both P. M. C. 

 and E. M. C, the following provisional assumption is made. First, 

 that those which appear to have divided in the equatorial region 

 and whose products actually are approaching the opposite poles 

 represent undivided univalents which, in the first division, started 

 to divide near one or other pole but were included entire in a 

 daughter nucleus. Second, that the, usually much smaller, chro- 

 mosomes which are merely segmenting in the equatorial region, or 

 out of it, represent the halves ol univalents which fully divided and 

 separated in the first division and that, in this case, their division 

 in homotypic anaphase is not completed or, at least, that the 

 products do not enter into Ihe subsequent formation of major nuclei. 



The fäte of the chromosomes in the development of the embryo- 

 sacs is of great importancc in these hybrids because a few of the 

 embryo-sacs are fertile and give viable seeds when crossed back 

 to the parents. The results of Ihese back-crosses are fully described 

 elsewhere (Univ. Calif. Publ. Botany, vol. 11, no. 1) but may be 

 summarized in very general fashion as follows. When sijloestris, 

 for example, is crossed back to F^ the j)rogeny contains plants 

 equivalent to sijloestris and also many abnormal forms. With 

 Tabacum the back-cross gives Tabacum, the Fj and abnormal 

 forms. The "extracted" parents are fertile and brced true, the 

 remainder of the populations is largely sterile. 



The cytological evidence presented above is interpreted as in- 

 dicating that the 12„ + 12, chiomosomes present represent 12 sijl- 

 nestris + 12 Tabacum and 12 unpaired Tabacum chiomosomes. 

 With this explanation in mind and in vicw of the genetic evidence 

 summarized above it is suggested that a few of the embryo-sacs 

 capable of fertilization are matured with the pure set of 12 sijl- 

 uestrls chromosomes, others with the pure set of 24 Tabacum chio- 

 mosomes and a larger number with variously contaminated parent 

 sets in the significant nuclei. The theoretical difficulties involved 



