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 numl^er of the chromosomes takes 

 place are extraordinarily difhcult to interi)ret, be- 



