284 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



EXPLANATION OF PLATE XXVIU. 



Fig. 25. In the upper half of the figure the chromosomes still preserve 

 their original arrangement; in the lower half they have already 

 become somewhat disarranged. The genetic fibres tend to col- 

 lect into strands which have not yet attached themselves to the 

 chromosomes. 



Fig. 26. The fibrous strands now show a more or less complete attach- 

 ment to the chromosomes and a tendency to assume a parallel 

 arrangement. 



Fig. 27. The strands have become compact fibres and are more nearly 

 parallel. 



Fig. 28. Transition to bipolar stage. In the lower half of the figure the 

 chromosomes are assembling in the equatorial region. 



Fig. 29. Completed spindle; not strictly bipolar, since the fusion of cones is 

 not complete. 



Fig. 30. Cross-section of spindle of first mitosis near the etjuatorial region, 

 showing the central core of connecting fibres interconnected by 

 delicate threads. 



