1002 REPORT—1896, 
It is now well known that in animals and in the higher plants a remarkable 
numerical change takes place in the constituents of the nucleus shortly before the 
act of fertilisation. The change consists in the halving of the number of chromo- 
somes, those rod-like bodies which form the essential part of the nucleus, and are 
regarded by Weismann and most biologists as the bearers of hereditary qualities. 
Thus in the lily the number of chromosomes in the nuclei of vegetative cells is 
twenty-four ; in the sexual nuclei, those of the male generative cell and of the ovum, 
the number is twelve. When the sexual act is accomplished the two nuclei unite, 
and so the full number is restored and persists throughout the vegetative life of 
the next generation. The absolute figures are of course of no importance; the 
point is, the reduction to one half during the maturation of the sexual cells, and 
the subsequent restoration of the full number when their union takes place. I say 
nothing as to the details or the significance of the process, points which have 
been fully dealt with elsewhere, votably in an elaborate recent paper by Miss E. 
Sargant. 
Now, in animals (so far as I am aware) and in angiospermous plants the reduc- 
tion of the chromosomes takes place very shortly before the differentiation of the 
sexual cells. Thus in a lily the reduction takes place on the male side immediately 
prior to the first division of the pollen mother-cell, so that four cell-divisions in all 
intervene between the reduction and the final differentiation of the male generative 
cells. On the female side the reduction in the same plant takes place in the 
primary nucleus of the embryo-sac, so that here there are three divisions between 
the reduction and the formation of the ovum. I believe these facts agree very 
closely with those observed in the animal kingdom, and so far there is no par- 
ticular difficulty, for we can easily understand that if the number of chromosomes 
is to be kept constant from one generation to another, then the doubling involved 
in sexual fusion must necessarily be balanced by a halving. 
There are, however, a certain number of observations on Gymnosperms and 
archegoniate Cryptogams which appear to put the matter in a different light. 
Overton ! first showed that in a Cycad, Ceratozamia, the nuclei of the prothallus or 
endosperm all have the half-number of chromosomes. Here then the reduction 
takes place in the embryo sac (or rather its mother-cell), but a great number of 
cell-generations intervene between the reduction and the maturation of the ovum. 
In fact the whole female odphyte shows the reduced number, while the sporophyte 
has the full number. The reduction takes place also in the pollen mother-cell. 
Further observations have extended this conclusion to some other Gymnosperms. 
In Osmunda among the ferns there is evidence to show that reduction takes 
place in the spore mother-cell, and that the sexual generation has the half-number 
throughout. Professor Farmer has found the same thing in various liverworts, 
and shown that the reduction of chromosomes takes place in the spore mother-cell ; 
and his observations of cell-division in the two generations have afforded some 
direct evidence that the odphyte has the half-number and the sporophyte the full 
number throughout. Professor Strasburger fully discussed this subject before 
Section D at Oxford,? and came to the conclusion that the difference in number 
of chromosomes is a difference between the two generations as such, the sexual 
generation being characterised by the half-number, the asexual by the full number. 
The importance of this conception for the morphologist is that an actual 
histological difference appears to be established between the two generations; a 
fact which would appear to militate against their homology. Some botanists even 
xo. so far as to propose making the number of chromosomes the criterion by which 
the two generations are to be distinguished. Considering that the whole theory 
rests at present on but few observations, I venture to think this both premature 
and objectionable ; for nothing can be worse for the true progress of science than 
to rush hastily to deductive reasoning from imperfectly established premises. 
The facts are certainly very difficult to interpret. Those who accept the 
antithetie theory of alternation suppose the sexual generation to be the older, and 
1 Annals of Botany, vol. vii. p. 139. 
2 See Annals of Botany, vol. viii. p. 281 
