156 THE CHROMOSOMES 
cause at this time the chromosomes are in the form 
of what seems to be a dense tangle of long threads. 
When this stage has been passed through, and the 
chromosomes are distinguishable again, the pairing 
has been completed. For any information that is 
worth while we have to rely on the best material 
available. It may be disputed which material is the 
best, but it will be generally conceded that a few 
types have shown themselves superior to others. 
The account of maturation that is here followed 
confines itself to two types—one for the male and the 
other for the female. These are selected cases, it is 
true, but they are those that give, in the opinion 
of the writers, two of the most complete accounts of 
these stages. The selection is admittedly not with- 
out bias, for these types can be most advantageously 
utilized to illustrate how crossing over can take 
place between the members of homologous pairs of 
chromosomes. 
The salamander, Batracoseps attenuatus, has fur- 
nished some of the best material for the study of the 
ripening of the germ cells of the male. The account 
that follows is taken from Janssens’ elaborate and 
detailed study of the spermatogenesis of Batracoseps. 
At the end of the multiplication period (spermato- 
gonial divisions) the nucleus appears as shown in 
Fig. 48,a. It then passes into a condition resembling 
a resting stage, b. Later the chromosomes begin to 
emerge in the form of long thin threads as shown in 
c, d, e. In the last figure (the leptotene stage) the 
ends of the thin threads are directed toward one pole 
