THE CHROMOSOMES 155 
seem never to re-appear subsequently, while those 
not eliminated always re-appear at the next cell 
division. Other cases of the same sort are known. 
In general it may be said that even an abnormal set 
of chromosomes, once established in a cell, tends to 
persist through all succeeding cell generations. This 
evidence indicates that the chromosomes are not 
mere products of the rest of the cell but are self- 
perpetuating structures. 
THE CHROMOSOMES DURING THE MATURATION OF 
THE GERM CELLS 
On the most essential point concerning the matura- 
tion of the egg and sperm there is no dispute: the 
observed number of chromosomes is reduced to half. 
It is generally agreed that this lowering of the number 
is due to the union of similar chromosomes in pairs, 
each chromosome derived from the father conjugating 
with the homologous chromosome derived from the 
mother. In cases where different chromosomes can 
be distinguished by their shape or size relations, the 
relations of these pairs correspond exactly to what 
they should be if like chromosomes conjugated. 
When we come to consider how this union of 
chromosomes is brought about, there is much diver- 
gence of opinion, for the evidence is fragmentary or 
contradictory on almost every point. The reason 
for this uncertainty is clear: the stages at which the 
reduction in the number of the chromosomes takes 
place are extraordinarily difficult to interpret, be- 
