468 BOTANICAL GAZETTE [june 



membrane" will be used. Undoubtedly the nuclear membrane, which 

 is the inner limiting layer of the cytoplasm formed there by the con- 

 tact of cytoplasm and karyolymph, is a semipermeable membrane, 

 and also, at the time of spindle formation, exosmosis is taking place, 

 since the nuclear cavity is gradually decreasing in size (figs. 22-27) 

 from the time of development shown in figs. 22 and 23, when the 

 fibers are first evident outside the membrane. Since the membrane 

 results from the contact of cytoplasm and karyolymph, the same 

 chemical reaction would occur when the nuclear sap gradually 

 diffuses into the cytoplasm. Such is the author's interpretation 

 of the weft of fibers outside the nucleus. The diffusion is gradual, 

 and the precipitation would then be slight, resulting in the fine 

 fibers. The cytoplasm is not forced to occupy more cubical space, 

 as Lawson claims, for karyolymph is steadily, though slowly, pass- 

 ing through this semipermeable membrane, and owing to the 

 precipitation the fibers occupy the space left by the receding nucleus. 



It is while the foregoing is occurring that the third contraction 

 or massing of the chromosomes in the center of the cavity becomes 

 so conspicuous (figs. 23-27), although as yet the fibers are not in 

 contact with them; and furthermore, the membrane disappears 

 previous to the filling up of the cavity by the fibers (figs. 24-26). 

 If the plasma membrane completely invested the bivalents, as 

 claimed by Lawson, a cavity would be left between it and the 

 fibers in the case of Allium tricoccum (figs. 25, 26), under which cir- 

 cumstances the fibers could not be moored to the membrane 

 (Lawson 20). 



It was owing to the contact of the two chemically different 

 substances that the heavy plasma membrane was precipitated, but 

 with the kinoplasmic fibers now being formed between the mem- 

 brane and the reticulate cytoplasm, this chemical antithesis is 

 decreased, resulting in a decrease in the amount of precipitation. 

 In other words, the membrane becomes thinner and thinner until 

 it either disappears entirely or becomes a part of the spindle fibers. 

 When the nuclear cavity is completely filled with the kinoplasmic 

 threads, the chromosomes loosen up, probably owing to the com- 

 pletion of exosmosis from the cavity, as well as partially on account 

 of the osmotic systems within each chromosome. 



