of the Pollen in certain Sweet-Peas. 153 



through before degeneration sets in. The first signs of irregularity 

 appear in the formation of the spireme thread ; the chromatin is 

 distributed unevenly, in such a way that portions of the thread 

 are destitute of that substance. The nucleolus generally becomes 

 irregularly lobed and is often connected with those portions of 

 the spireme which contain chromatin by prominent connecting 

 threads. 



The longitudinal fission of the spireme is effected, at any rate 

 in those parts of the thread in which chromatin is present (Fig. 9). 

 The thread now becomes very irregular, both in structure and in 

 configuration, and has a tangled appearance (Figs. 10 — 12) ; any 

 indications of looping which may be present show corresponding 

 irregularities, while the threads containing the chromatin are 

 connected with one another to a greater or less degree by irregular 

 threads from which chromatin granules are absent. In some cases 

 the thread now undergoes a partial segmentation, indicating the 

 attempted formation of chromosomes (Fig. 13). In others the 

 chromatin aggregates into irregular masses which are connected 

 with one another by faintly stained threads (Figs. 14, 15). It 

 should be mentioned that these masses generally number about 

 eight, and sometimes give the appearance of being double. The 

 process rather recalls that of the shortening and thickening of the 

 chromosomes of fertile cells just before metaphase. 



These changes are accompanied by a drawing-out of the whole 

 nucleus, sometimes in the direction of more than two poles 

 (Fig. 13) ; at this time the nuclear walls disappear, and in some 

 cases the development of a few striae at the poles of the nucleus 

 indicates an attempted formation of the spindle (Fig. 13). 



From this condition almost every form transitional to the 

 curious state shown in Figs. 16 and 17 may be found. In the 

 earlier stages of this form of abnormality, it is quite clear that 

 the masses of deeply staining matter, which are, as it were, left 

 behind by the drawing-out and constriction of the nucleus, consist 

 of a closely tangled thread. 



In the later stages of all forms the anthers contain only a 

 matrix in which the cell outlines become less and less distinguish- 

 able ; embedded in this lie rounded or lobed masses which stain 

 deeply, and, by comparison with the earlier stages, doubtless 

 represent the degenerating nuclei. 



The sterility in these plants is confined to the male organs*. 

 The structure of all the embryo-sac mother-cells examined has 

 been quite regular, the contraction of the spireme and the forma- 

 tion of loops resembling exactly that of the fertile plants 

 (Fig. 4). 



* See also Bateson, Saunders, and Punnett, I.e., p. 91. 



