MATURATION. 19 



considered as chromosomes, the question arises as to the relation that the rod- 

 like elements composing the tetrads bear to the chromatin of the nucleus before 

 the tetrads are formed. In other words, what is the origin of the tetrads ? The 

 answer to this, even in Ascaris, is not quite clear. It is generally agreed, how- 



z.p. 



FIG. 10. From section of ovum (primary oocyte) of the mouse^showing first maturation 

 spindle. Note the 12 chromatin segments, the somatic number of chromosomes being 24. The 

 ovum is surrounded by the zona pellucida (z.p.) and the corona radiata. Sobotta. 



ever, that after the formation of the spireme thread, the latter segments into 

 two chromatin rods instead of four; that is, it segments into one-half the usual 

 number of pieces. Each piece splits longitudinally and then each resulting 

 piece splits longitudinally again. Thus each primary segment gives rise to 



FIG. ii. From sections of ova of the mouse. The figure on the left shows a tangential matu- 

 ration spindle with 12 chromatin segments. The figure on the right shows a tangential maturation 

 spindle in which each of the 12 chromatin segments has divided transversely into two equal parts, 

 thus forming 24 segments. Sobotta. 



four segments which constitute a tetrad. From this it is evident that the 

 crucial point in reduction is the segmentation of the spireme thread. 



In Ascaris, as described above, reduction takes place by means of tetrad 

 formation, the tetrads being formed by a double longitudinal splitting of the 



