Ch. 2 — Introduction • 31 
beginning ot the 20tli centiirv did scientists 
rediscover Mendel’s work and begin to ap[)re- 
ciate fully the significance of the cell nucleus 
and its contents. 
K\en in the earliest microscopic studies, 
boue\er, certain cellular com[)onents stood 
out; they were deeply stained by added dye. As 
a result, they were dubbed “coloretl bodies,” oi' 
chromosomes. Chromosomes v\ere seen rela- 
ti\ely rarely in cells, with most cells showing 
just a central tlark nucleus surrounded by an 
e.xtensive light grainy cytoplasm. But periodi- 
cally the nucleus seemed to disappear, leaving 
in its place long thready material that con- 
solidated to form the chromosomal bodies. (See 
figure 6a.) Once formed, the chromosomes 
assembled along the middle of the cell, copied 
themsek es, and then mo\ ed apart w bile the cell 
pinched itself in half, trapping one set of 
chromosomes in each of the two hakes. I hen 
the chromosomes themsekes seemed to dis- 
soke as two new nuclei appeared, one in each 
of the tw o newly formed cells. (See figure 6h.) 
Thus, the same number of chromosomes ap- 
peared in precisely the same form in e\ery cell 
of an organism e.xcept the germ, or sex, cells. 
Furthermore, the chromosomes not only re- 
mained constant in form and number from one 
generation to the next, hut were inherited in 
pairs. They were, in short, manifesting all the 
traits that Mendel had prescribed for genes 
almost three decades earlier. By the beginning 
of the 20th century, it was clear that chromo- 
somes w'ere of central importance to the life his- 
tory of the cell, acting in some unspecified man- 
ner as the vehicle for the Mendelian gene. 
If this conclusion was strongly implied by the 
e\ ents of cell di\ ision, it became obvious when 
I'eproduction in whole organisms was analyzed. 
It had been established by the latter part of the 
19th century that the germ cells of plants and 
animals— |)ollen and o\ um, sperm and egg— ac- 
tually fuse in the [process of fertilizaton. Germ 
cells differ fi’om other body cells in one impor- 
tant resj)ect— they contain only half the usual 
number of chromosomes. This chromosome 
baking within the cell was apparently done 
\'ery precisely, for e\'ery sperm and egg con- 
tained exactly one representative from each 
chromosome pair. When the two germ cells 
then fused during fertilization, the offspring 
were supplied with a fully I’econstituted chro- 
mosome complement, half from each parent. 
C^learly, chromosomes were the material link 
from one generation to the next. Somewhere 
locked within them was the substance of both 
heredity— the fidelity of traits between genera- 
tions; and diversity— the potential for genetic 
\ ariation and change. 
