OOLOGY AND BOTANY, MICROSCOPY, ETC. 731 



BOTANY. 



GENERAL, 

 Including the Anatomy and Physiology of Seed Plants. 



Cytology, 

 including' Cell Contents. 



Studies in Spindle Formation.* — A. A. Lawson gives the result of 

 his studies in this subject. Previous observations showed that there is 

 considerable variety in the method of spindle-formation, and it would 

 seem that there are several distinct types ; but the differences between 

 them are too great, and the number of forms worked out in detail are 

 too few, to allow of any generalisations. For his present paper the 

 author has studied the pollen mother-cells of Iris florentina, of Disporum 

 Hookeri, of Hesperaloe Davyi, and of Hed&ra Helix. In Iris, formation 

 of the spindle is initiated by the transformation of the cytoplasmic 

 reticulum close to the nuclear membrane into a weft of kinoplasmic 

 fibrils, which forms a complete zone about the nucleus. After increasing 

 to a certain thickness, the zone projects outwards at irregular intervals, 

 forming a series of sharply pointed cones, which apparently develop at 

 the expense of the cytoplasmic reticulum into which they project, and as 

 they grow the fibrils composing them lengthen and converge at the 

 apex. The nuclear wall persists until the completion of the cones, which 

 fuse on the breaking down of the membrane until there are two groups 

 of them pointing in opposite directions. The points at which the cones 

 forming these groups meet at their apices become the poles of the bipolar 

 spindle. 



In Disporum the first indication of the spindle is the formation of a 

 weft of kinoplasmic fibrils partially surrounding the nucleus, and formed 

 from the cytoplasmic membrane, but, unlike Iris, the meshes of the weft 

 do not run parallel to the nuclear membrane. The weft increases 

 irregularly, forming several projections which become the primary cones 

 of the spindle. As they grow outward the cones become sharp-pointed, 

 and their fibrils are sharply defined. The nuclear wall then breaks down, 

 and the cones unite in two groups to form the bipolar spindle. 



As in Iris and Disporum, the spindle in Hesperaloe originates from a 

 weft of kinoplasm, which completely surrounds the nucleus and is of 

 cytoplasmic origin ; the fibrils of the weft run parallel to the nuclear 

 membrane. The weft develops a series of sharp-pointed projections 

 which become the primary cones of the spindle. As the nuclear wall 

 disappears the cones collect in two groups, and fusion at their apices 

 effects the bipolar condition. In Hedera the cytoplasm close to the 

 nucleus becomes changed into a weft of kinoplasmic fibrils, surrounding 

 the nuclear membrane ; the zone forms a number of sharp projections 

 which become the primary cones of the spindle. As these cones grow 



* Bot. Gazette, xxxvi. (190:!) pp. Sl-100 (2 double pis.) 



