ZOOLOGY: T. H. MORGAN 
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gray or else red yellow. In other words an exchange of characters has 
in some way taken place, but not oftener than once in a hundred times. 
We can see the exchange taking place in the reverse way if we cross a 
white eyed, gray winged fly to a red eyed, yellow winged fly. In the 
second generation the flies will now be white gray and red yellow, except 
that once in a hundred times a fly will appear that is white yellow or 
red gray. 
Now if at the time when the chromosomes conjugate the two Hke 
chromosomes should exchange pieces, a consistent explanation of the 
results described above can be given. As a matter of fact there are 
several stages before, during and after conjugation of the chromosomes 
when interchange of pieces might take place. One of the phases is il- 
JJK JJK JJK 
FIG. 4. DIAGRAMS OF THE CHROMOSOMES OF DROSOPHILA AMPELOPHILA: TO THE 
LEFT, NORMAL FEMALE; IN THE MIDDLE, NORMAL MALE; TO THE RIGHT, NON-DISJUNC- 
TIONAL FEMALE. 
lustrated in the following diagram (fig. 5). Two homologous chromo- 
somes are represented as each made up of a single line of beads. Dur- 
ing conjugation, like chromosomes come together, and as they unite 
they can be seen often to twist around each other in consequence of 
which the parts come to line up on opposite sides of each other, as shown 
in the figure. When the chromosomes move apart the beads on one 
side are represented as passing to one pole, and those on the opposite 
side to the other pole ; this means that during conjugation the pairs of 
factors, here represented by the beads, segregate independently of each 
other, and that the forces that draw the beads apart are often stronger 
than those that hold consecutive beads in their original line or series. 
It may then, I think, be fairly claimed that the facts of crossing-over 
which must be due to interchange between homologous chromosomes, if 
chromosomes be admitted as the bearers of the hereditary factors, can 
be accounted for without departing from the known behavior of the 
chromosomes at the time of conjugation. On the other hand it should 
be clearly understood that the conclusion as to the way in which the 
