PLATE B 



Minnesota 3 : 



1. — Pollen mother cell previous to synapsis. X 2,000. 



2. — Presynapsis in the pollen mother cell showing loops extending out from 

 synaptic mass. X 2,000. 



3. — ^Two loops and portion of a loop extending from the presynaptic mass. Same 

 stage as figure 2. X 3,280. 



4. — Synapsis in a pollen mother cell. X 2,000. 



5. — A postsynaptic stage. The synaptic mass is unraveling into bivalent loops. 

 X 2,000. 



6. — Open spireme stage. X 2,000. 

 7. — A presegmentation stage of the spireme. X 2,000. 



8. — A portion of a bivalent spireme thread of the same stage as figure 7. X 3)28o. 

 9. — Segmentation of the bivalent spireme into chromosome pairs. The cytoplasm 

 is roxmding up and is partially surrounded by a gelatin-like sheath. X 2,000. 

 10. — A portion of the bivalent spireme during segmentation. X 2,340. 

 II, 12. — Chromosome pairs during the contraction period following segmentation. 

 X 2,000. 



13. — Individual chromosome pairs showing various figures commonly formed during 

 contraction. X 3.280. 



14. — Diakenesis in the pollen mother cell. X 2,000. 

 15. — Multipolar spindle stage of pollen mother cell. X 2,000. 

 16. — Early anaphase of the heterotypic division. X 3,280. 



(670) 



