Sexual Nuclei in Lilium Martagon . 447 
the plant arose. It is impossible however to distinguish 
those nuclear divisions which are in the direct line of descent 
— the c germ track ’ — from those which are not, until we are 
within a few generations of the sexual nuclei. In the young 
anther as soon as the archesporial tissue is differentiated we 
can say with certainty that all the nuclei within it will by 
repeated divisions give rise to pollen mother-cell nuclei, and 
therefore four generations later to the male nucleus of the 
pollen-tube. The ancestry of the ovum cannot be traced 
back so far. The embryo-sac is a hypodermal cell, distin- 
guished at first from those surrounding it by its median 
position only. Three successive nuclear divisions within the 
embryo-sac divide its primary nucleus from the ovum, and 
only during these three generations can the line of descent be 
securely followed. 
Fortunately it is not necessary for our present purpose that 
the divisions which will form the ovum should be identified 
through more than three generations, nor those which are to 
form the male generative nucleus through more than four. 
For it is well known that the three karyokinetic divisions 
which immediately precede the formation of the ovum, and 
the four preceding that of the male generative nucleus, differ 
from their predecessors in possessing twelve chromosomes in 
place of twenty-four. These twelve chromosomes appear 
in the primary embryo-sac nucleus and in that of the pollen 
mother-cell respectively just before the formation of the 
spindle. According to Dr. Haecker’s hypothesis, each of the 
twelve represents two of the chromosomes from the previous 
division joined end-to-end. The reduction in number implies 
no corresponding reduction in mass, for the twelve new 
chromosomes are in fact twelve double segments, equivalent 
to the twenty-four single segments of the previous karyoki- 
nesis. If the subsequent karyokineses effect a longitudinal 
fission of each chromatic segment as botanists have hitherto 
asserted, then each segment which goes to build up the 
nucleus of the ovum or the male generative nucleus is a de- 
scendant of one of these double chromosomes and of similar 
