286 S Iras burger . — The Periodic Reditction of 
on plants, I have 1 , from the very first, maintained the view 
that the reduction in the number of chromosomes in the 
generative nuclei is not effected by the extrusion of some of 
them during the ripening of the sexual cells, but that, on the 
contrary, the development of the sexual cells depends on 
indirect nuclear division with longitudinal splitting of the 
chromosomes ; and these views are also held by many 
observers as regards animals, and by Guignard 2 as regards 
plants. Guignard and I 3 established the fact that the number 
of chromosomes characteristic of the generative nuclei of 
Angiosperms, is determined, in the one case, in the mother- 
cells of the pollen, and, in the other case, in the mother-cells 
of the embryo-sacs. The investigations of the zoologists 
have also shown that this determination is effected in the 
mother-cells of the ova and of the spermatozoa of animals, by 
two successive cell-divisions, which give rise, in the one case, 
to four spermatozoa, and, in the other, to the ovum and 
to three so-called polar bodies. Guignard 4 in particular 
has endeavoured to follow with absolute accuracy all the 
processes affecting the reduction of the number of the chromo- 
somes in the pollen-sacs and ovules of Lilies. The reduction 
takes place directly, both in the mother-cells of the pollen 
and in the mother-cell of the embryo-sac, and in such a manner 
that the reduced number of chromosomes is at once apparent 
in the prophase-stage. In all the preceding nuclear divisions, 
both in the pollen-sac and in the ovule, twenty-four chromo- 
somes are, almost uniformly, visible : the framework of the 
resting-nucleus of the pollen-mother-cell and of the embryo- 
1 Neue Unters. iib. den Befruchtungsvorgang, 1884, pp. 16, 82 : Ueb. Kern- und 
Zell-Theilung, 1888, p. 232. 
2 Etudes sur les phenomenes morphol. de la fecondation, Bull, de la Soc. Bot. 
de France, XXXVI, 1889, p. 106, &c. 
3 Strasburger, Ueb. Kern- und Zell-Theilung, 1888, pp. 51 and 240 ff. : Guignard, 
loc. cit., p. 105 ff. ; and Nouvelles Etudes sur la Fecondation, Ann. Sci. Nat., Bot., 
Ser. 7, t. XIV, 1891, p. 246 ff. : compare also, Overton, Beitr. z. Kenntniss der 
Geschlechtsproducte des Lilium Martagon , Festschrift fur Kolliker und Naegeli, 
1891. 
4 Nouvelles Etudes, pp. 173, 182. 
