94 



CELL-DIVISION 



the diatoms {Stirirella, t. Lauterborn), where the central spindle, 

 arising by a peculiar process from an extranuclear centrosome, 

 (sphere ?) sinks into the nucleus in a manner strongly suggesting that 

 observed in Noctiluca. 



In the interesting form Pamuiaba, as described by Schaudinn 

 ('96, I), the sphere (" Nebenkorper "), which is nearly as large as the 

 nucleus, divides to form a central spindle, about the equator of which 

 the chromatin-elements become arranged in a ring (Fig. 43); but no 

 centrosome has yet been demonstrated in the sphere. Paramaba 

 appears to differ from Euglcna mainly in the fact that at the close of 

 division the sphere is in the former left outside the daughter-nucleus 

 and in the latter enclosed within it.^ The connecting link is perfectly 

 given by Teiramitus, where no morphological nucleus is formed, and 

 the sphere lies in the general cell-substance (p. 92); and we could 

 have no clearer demonstration that the extra- or intranuclear position 

 of sphere or centrosome is of quite secondary importance. As 

 regards the formation of the spheres (pole-plates) ActinospJicerinm 

 (Figs. 44, 45) seems to show a simpler condition than any of the 

 above forms, since no permanent sphere exists, and Brauer ('94) and 

 R. Hertwig ('98) agree that the pole-plates are formed by a gradual 

 accumulation of the achromatic substance of the nucleus at opposite 



poles. 



A distinct centrosome (centriole }) in the interior of the sphere has 

 thus far only been observed in a few forms {Noctiluca, ActiiwsphcB- 

 riitm), and neither its origin nor its relation to the sphere has yet 

 been sufficiently cleared up. Both Ishikawa ('94) and Calkins ('98, 2) 

 somewhat doubtfully concluded that in Noctiluca the centrosomes 

 arise within the nucleus, migrating thence out into the extranuclear 

 sphere. With this agree R. Hertwig's latest studies on ActinospJi(B- 

 rinm ('98), the spindle-poles being first formed from the pole-plates 

 (themselves of nuclear origin), and the centrosomes then passing into 

 them from the nucleus. Hertwig reaches the further remarkable 

 conclusion that the centrosomes arise as portions of the cJiromatin- 

 network extruded at the nuclear poles (Fig. 45), first forming a 

 spongy irregular mass, but afterward condensing into a deeply 

 staining pair of granules which pass to the respective poles of the 

 spindle. It is a remarkable fact that these centrosomes are only 

 found in the two maturation-divisions, and are absent from the ordi- 

 nary vegetative mitoses where the spindle-poles are formed by two 

 cytoplasmic masses derived, as Hertwig believes, from the intra- 

 nuclear plates. Schaudinn ('96, 3) likewise describes and clearly 

 figures an intranuclear origin of the centrosome in buds of Acantho- 

 cystis (Fig. 41), which are derived by direct division of the mother- 



1 Cf. Calkins, '98, i, p. 388. 



