No. 450.] STUDIES ON THE PLAWT~1?ELL. 437 



nuclear spindles) are of two main types : (i) those associated 

 with centrosomes, centrospheres or kinoplasmic caps, and (2) 

 those composed of independent fibrillae developed as a mesh 

 around the nucleus. The latter condition is especially character- 

 istic of the spore mother cell and is perhaps the highest type of 

 spindle formation known for either animals or plants. It is very 

 interesting to trace the relations of this highest condition to the 

 lower types through certain lines of evolution to be discussed in 

 Section VI. 



Spindles with centrosomes are known in Sphacelaria, Stypo- 

 caulon (Swingle, '97), Dictyota, Fig. 4 a (Mottier, : oo), the zoo- 

 sporangium of Hydrodictyon (Timberlake, : 02), in certain 

 diatoms (Lauterborn, principal paper '96, Karsten, : oo) and in 

 the basidium (Wager, '94 and Maire, :O2). The best accounts 

 of the behavior of the centrosomes are given by Swingle and 

 Mottier. Indeed there is much doubt about the history and 

 significance of the bodies in the other forms, although the con- 

 stancy of their presence at the poles of the spindles indicates 

 that they are really centrosomes. The conditions in the diatoms 

 are especially complicated ; an account of Lauterborn's work has 

 been published in English by Rowley, :o3. In Stypocaulon, 

 Sphacelaria (Fig. 3 c, Section I) and Dictyota (Fig. 4 a) the cells 

 studied have permanent asters which lie at the side of the 

 nucleus and which divide just previous to the mitosis and sep- 

 arate so that they come to lie on opposite sides of the nucleus. 

 Fibers develop from the centrosomes on the sides nearest the 

 nucleus and elongating push against the nuclear membrane and 

 finally enter the nuclear cavity to form the spindle. 



Spindles with centrospheres are well known in Fucus (Farmer 

 and Williams, '96, '98, Strasburger, '9/a), Corallina, Fig. 5 c, 

 (Davis, '98), in the ascus, Fig. 5 b (Harper, '97 and '99), and in 

 the germinating spore of Pellia, Fig. 4 c (Farmer and Reeves, 

 '94, Davis, :oi, Chamberlain, :O3). Centrospheres have been 

 reported in other forms but the types mentioned above have 

 received the most careful study. It is probable that the centro- 

 sphere is but a larger, more generalized kinoplasmic center than 

 the centrosome, a protoplasmic region whose dynamic activities 

 do not focus so sharply as in the latter structure. There are 



