and Plants. Reducing Division in Metazoa. 449 
unite after the conjugation, until the first division of the 
zygote shall have happened, is again an indication that 
a longitudinal splitting does not bring about a reduction. 
When actual union of chromosomes after conjugation is 
effected, this is obtained by the union of the chromosomes 
from an individual A with a corresponding number of chro- 
mosomes of an individual B ; the chromosomes must, as 
others have often enough insisted, retain their identity and 
they only become disunited for the purposes of a cell-division. 
Ultimately it becomes necessary to finally undo the linking, 
in order to prevent a duplication of the number which would 
increase it to fourfold what it originally was. This can only 
be effected by a transverse splitting. In other words, chro- 
mosomes may be considered as possessing two axes, along 
one of which (the longitudinal) they may divide, along the 
other (the transverse) they may unite with other chromosomes. 
It is along the latter — that along which union takes place — 
that the reduction must be effected. A reduction is nothing 
more than an undoing of the union effected at a previous 
conjugation, but by this it must not be concluded that there 
is any intention of supposing it to be a separation of all the 
male-parental chromosomes from all the female-parental ones. 
The facts of heredity, as Weismann has proved, go to show 
that the process is more complicated, and an excellent dis- 
cussion and explanation of it have been furnished by Haecker 
in his reply to Strasburger and elsewhere. The theoretical 
mode of the undoing has quite recently been proved by an 
able investigator in the case of the Copepoda 1 . 
Riickert has shown that in this group the reduced number 
is brought about by a transverse division of what seemed to 
be half the normal number of chromosomes, and that in the 
ripening of the egg this takes place in the formation of the 
second polar body. He states 2 that the reduction in the 
number of chromosomes before fertilization is attained by the 
1 Riickert, J., Die Chromatinreduction bei der Reifung der Sexualzellen. 
Ergebnisse der Anat. und Entwickelungsgesch., Bd. Ill, 1893, pp. 517-583. 
2 Summary on p. 582. 
