of Fertilization in Angiosperms. 705 
number of chromosomes. So far we have a typical sexual 
union. But the third element in the fusion upsets the balance. 
The lower polar nucleus, though originally derived from the 
first division of the embryo-sac nucleus, which is a reduction- 
division, has since acquired a larger number of chromosomes. 
In Lilinm Martagon , for example, the two male generative 
nuclei—the nucleus of the egg-cell, and the upper polar nucleus 
—are each built up of 13 chromosomes. The lower polar 
nucleus, however, is built up of at least 24 chromosomes, 
sometimes of more. (Guignard, 23 , pp. 187-8 ; Sargant, 37 , 
pp. 465 and 468.) 
The nuclear fusion then which produces the first endosperm- 
nucleus differs from the normal conjugation of the sexual 
nuclei by the presence of a large mass of chromosome- 
substance derived from a nucleus which shows no sexual 
differentiation. The fusion is certainly accompanied by a 
growth-stimulus, for the compound nucleus divides very 
rapidly. 
In the sea-urchin (Boveri, 18 , pp. 425-6) more than one 
sperm-nucleus can be forced to unite with a single egg-nucleus. 
Compound spindles are then formed, and the larval structures 
derived from the division of such ova are monstrous. It seems, 
however, that a normal larva is sometimes produced as a 
consequence of the union of two sperm-nuclei with one egg- 
nucleus. In such a case the number of chromosomes in the 
nucleus of the fertilized ovum would be one-half in excess 
of the usual somatic number, or three times the reduced 
number. But the first endosperm-nucleus of Lilium Martagon 
contains at least twice the somatic number of chromosomes, 
or four times the reduced number. Many observers have 
noticed the large number of chromosomes characteristic of 
dividing nuclei in endosperm-cells. 
The primary endosperm-nucleus formed by the triple 
nuclear fusion just discussed, gives rise on division, not to 
a normal embryo, which will pass through all the stages in 
the life-history of its species, and ultimately grow into a com¬ 
plete plant, but to an endosperm, which is a comparatively 
3 A 2 
