Pollen Mother -cells of Lilium canadense. 233 
notable exceptions to this rule are known, in the form of differences between 
reciprocal crosses, e. g. the mule and hinny, Gartner’s and Focke’s (Swingle 
and Webber, ’98) Digitalis crosses, the Nymphaea hybrids of Caspary, 
and some of the grape hybrids studied by Millardet. 
In the foregoing paragraphs I have attempted to point out what seems 
to me the remarkable parallel existing between the data obtained by the 
experimental study of plant and animal breeding on the one hand, and, on 
the other, such results of cytological investigation as the constancy of the 
chromosome number, the continuous separation between the parental 
elements within the nuclei of the offspring, the individual persistence and 
distinctive appearance of the chromosomes, and the finer details of their 
structure. I shall return later to a consideration of the experimentally- 
derived facts and of the light that they may throw upon the problems with 
which the present paper is especially concerned. 
The Reduction of the Chromosome Number. 
The conception of the individuality of the chromosomes, together with 
the fact of the general constancy of their number in any particular species, 
involve, as Boveri (’88, ’90) early pointed out, the necessity, at some period 
in ontogeny, of a reduction of their number to one-half, in order that that 
number may not be doubled in each generation by the fusion of the sexual 
nuclei. We now know that such a reduction does occur at a definite 
point in the life-history of every sexually-produced individual, at least 
among the higher animals and the higher plants ; that in some way two 
successive nuclear divisions, differing from the ordinary type, are concerned 
in this numerical reduction ; and that the reduced number of elements 
(whether at this stage involving a real or only an apparent reduction) is first 
to be observed in the prophases of the earlier of these two peculiar divisions. 
We know, too, that in the Metazoa the divisions concerned in chromosome 
reduction are the two immediately preceding the formation of the definitive 
sexual cells ; and that in the Spermaphytes, Pteridophytes, and Bryophytes, 
these divisions are the two which result in spore-formation, and which, 
therefore, determine the transition from the sporophyte to the gametophyte 
generation. 
The prophases of the first of the tw T o divisions just referred to (the 
heterotypic division) are characterized by their unusually long duration, 
and by the appearance of a stage in which the nuclear constituents are 
aggregated into a more or less compact mass, usually in contact with one 
side of the nuclear membrane. This stage of aggregation, to which Moore 
(’95 a) gave the name ‘ synapsis,’ is regularly found, so far as we now know, 
only in the prophases of this division. Although this condition had been 
described by various writers, Brauer (’93) seems to have been the first to 
