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. 43, a. It then passes into a condition resembhng 

 a resting stage, h. 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 



