Chromosomes in Living Organisms . 289 
a return to the original generation from which, after It had 
attained sexual differentiation, offspring was developed having 
a double number of chromosomes. Thus the reduction by one- 
half of the number of the chromosomes in the sexual cells is 
not the outcome of a gradually evolved process of reduction, 
but rather it is the reappearance of the primitive number of 
chromosomes as it existed in the nuclei of the generation in 
which sexual differentiation first took place. Viewed from 
this standpoint, many facts become more readily intelligible : 
for instance, the Immediate and sudden occurrence of the 
reduction, the developmental stage at which it takes place, 
and the varying length of the interval which separates It from 
the sexual act. 
The number of chromosomes determined in the mother- 
cells of the pollen of the Angiosperms persists up to the 
formation of the spermatic nucleus in the pollen-tube. Four 
divisions are involved in the development of this nucleus : 
two divisions take place in the mother-cell resulting in the 
formation of four pollen-grains ; then there is the division in 
the pollen-grain by which the generative and the vegetative 
cells are respectively formed ; and, finally, there is the fourth 
division, the division into two of the generative cell in the 
pollen-tube. The number of chromosomes determined in the 
mother-cell of the embryo-sac persists through a series of 
divisions, the number of which varies with the species of plant, 
until it attains functional importance in the ovum. As a rule, 
the mother-cell of the embryo-sac divides twice, and the 
lowest of the resulting daughter-cells develops into the 
embryo-sac. In the embryo-sac three divisions succeed each 
other before the nucleus of the ovum is formed. In this case 
five divisions, and not four as in the development of the sper- 
matic nucleus, intervene between the reduction and determi- 
nation of the number of the chromosomes, on the one hand, 
and the constitution of the sexually-functional nucleus on the 
other. That the number of these intervening divisions Is not 
of primary Importance, is proved by the fact that the number 
is not always the same : thus, in Liliitm and Tulip a there 
