igi2] SHARP— SPERMATOGENESIS IN EQUISETUM 103 



During karyokinesis it lies near the pole of the broad-poled 

 spindle. 



Centrosomes in the Sphacelariaceae have been described by 

 Strasburger (78), Humphrey (48), and Swingle (83). In the 

 vegetative cells of Sphacelaria the centrosome, according to Stras- 

 burger, is situated in a centrosphere at the center of an aster. 

 Previous to mitosis it divides to two which take up positions at 

 opposite poles. In Stypocaulon Swingle has shown that the 

 centrosome, which lies close to the nucleus, divides, the daughter 

 centrosomes diverging to opposite sides of the nucleus and occupy- 

 ing the spindle poles throughout mitosis. At all stages asters are 

 present. Swingle is inclined to regard this centrosome as a 

 permanent organ of the cell. 



In the oogonium and segmenting oospore of Fucus Farmer 

 and Williams (24, 25) described two centrospheres arising inde- 

 pendently 180 apart. In the centrosphere they often observed 

 several granules, but were inclined to attach no importance to 

 them. Strasburger (79) reported definite centrosomes with 

 asters all through karyokinesis; in the sporeling are stages which 

 indicate that it is a dividing body. He regarded it as a permanent 

 cell organ. In a more recent investigation Yamanouchi (98) 

 demonstrates in the antheridium and oogonium two very definite 

 centrosomes, which appear independently of each other, become 

 surrounded by conspicuous asters, and occupy the spindle poles 

 during karyokinesis. He further shows that when the sperm 

 reaches the egg nucleus a new centrosome appears on the nuclear 

 membrane at the spot where the sperm entered. 



The centrosome of Dictyota has been dealt with by two investi- 

 gators. Mottier (69, 70) states that in the two divisions in the 

 tetraspore mother cell, in at least the first three or four cell genera- 

 tions of the sporeling and in all the vegetative cells of the tetrasporic 

 plant, a curved rod-shaped centrosome with an aster occurs at the 

 spindle pole. During the early phases of karyokinesis it divides, 

 the daughter centrosomes passing to opposite poles. Williams 

 (91) figures centrosomes and asters essentially like those described 

 by Mottier. He also states that the entrance of the sperm causes 

 a centrosome with radiations to appear in the egg cytoplasm. 



