156 Mr Gregory, The Abortive Development 



Figs. 9-17 illustrate the later stages of sterile plants in which 

 degeneration of the pollen -mother-cells takes place before chromosomes 

 are formed. 



Fig. 9. Sterile Pea. Pollen-mother-cell showing the irregular 

 spireme, and its longitudinal fission, x 1600. 



Figs. 10, 11. Sterile Pea. Rather later stage in which the 

 spireme is somewhat tangled. The longitudinal 

 fission can still be traced. There seems some 

 indication of the polarity which is observed in fertile 

 cells, x 1600. 



Fig. 12. Sterile Pea. Showing the close tangling of the spireme. 

 The nuclear wall is only visible at the ends of the 

 poles of the rather elongated nucleus. The chromatin 

 thread is surrounded by an area of clear protoplasm. 

 The remains of the nucleolus, now staining very 

 faintly, lie in the centre of the nucleus and are sur- 

 rounded by a series of threads containing little 

 chromatin, but connecting with the tangled portions 

 of the chromatin thread, of which they really form a 

 part, x 1600. 



„ 13. From the same anther as Fig. 12. The nuclear wall 

 has disappeared. Some parts of the thread have 

 separated, representing the attempt to form chro- 

 mosomes. The longitudinal fission is visible in some 

 places. Cytoplasm of a definitely fibrous structure 

 surrounds the clear area. There is a suggestion of 

 multipolarity. x 1600. 



Plate II. 



Fig. 14. Sterile Pea. A later stage showing the aggregation of 

 the chromatin into masses. The nuclear wall has 

 disappeared. The nucleolus is still visible as a faintly 

 staining body, x 1600. 



,, 15. The same in a rather later stage. The deeply staining 

 bodies are connected with one another by slightly 

 stained threads, x 1600. 



Figs. 16, 17. Sterile Pea. The incomplete and abnormal division, 

 by constriction of the pollen-mother-cells, which occurs 

 in a few cases, x 1600. 



Fig. 18. "Emily Henderson." Metaphase of the heterotype di- 

 vision in the fertile form, x 1600. 



Figs. 19, 20. Sterile Pea. Spindles of irregular form such as occur 

 in those sterile plants which show least divergence 

 from the normal type, x 1600. 



