436 THE AMERICAN NATURALIST. [VOL. XXXVIII. 



spindles of Hydrodictyon are intranuclear or not ; they lie in a 

 clear space which, however, may be a vacuole rather than the 

 outline of a nuclear cavity. It seems probable in such a type 

 that the vacuole is really the nuclear cavity whose plasma 

 membrane (nuclear membrane) becomes less clearly defined. 

 The development of the spindle is very difficult to follow among 

 these lower forms because it is so small. Stevens (103) found 

 an exceptionally favorable type in Synchytrium and came to 

 the conclusion that the spindle developed from the threads of 

 the spirem (limn) entirely within and independent of the 

 nuclear membrane. 



Very remarkable intranuclear spindles have been described in 

 the central cell of the pollen tube of Cycas (Ikeno, '98 b) and 

 Zamia, Fig. 5d (Webber, :oi). Murrill (: oo) found them in 

 the mitosis following the fusion of gamete -nuclei in the egg of 

 Tsuga, Ferguson (:oib) at the same period for pine, and Coker 

 (: 03) in Taxodium. They are also reported by Strasburger (: oo) 

 in the cells of young anthers and nucelli of the lily and in grow- 

 ing points (Viscum) and possibly may be found quite generally 

 in cells weak in kinoplasmic cytoplasm. The development of 

 the spindles in the above forms has not been studied in detail, 

 but the fibers are probably derived from the linin. We are 

 given a clue to the process by the events of spindle formation in 

 the spore mother cell of Passiflora (Williams, '99). In this 

 angiosperm the nuclear cavity becomes filled with a fibrillar 

 network developed from the linin, the nuclear wall becomes 

 transformed into a mesh connecting the intranuclear fibers with 

 a surrounding cytoplasmic reticulum. The fibers in the central 

 region of this net work develop the spindle which is consequently 

 very largely of intranuclear origin. 



Among the thallophytes the poles of intranuclear spindles are 

 frequently occupied by deeply staining bodies which have been 

 called centrosomes ; but these structures can hardly be homol- 

 ogous with the well-known centrosomes of other thallophytes, 

 e. g., Stypocaulon (Swingle, '97) and Dictyota (Mottier, : oo). 

 They are probably merely temporary accumulations of material 

 with no morphological significance. 



Spindles that arise from fibers external to the nucleus (extra 



