82 



ixtkodcctiox to cytology 



Bryophytes.- -The first centrosome known in the liverworts was that 

 of Marchantia described by Schottlander (1893), according to whom the 

 centrosome in the spermatogenous cells divides during the anaphases of 

 mitosis, so that each daughter nucleus is accompanied by two (Fig. 27). 

 In the gametophytic colls certain minute bodies with radiations at the 

 poles of the elongated nucleus and of the spindle were believed by Van 

 Hook (1900) to represent centrosomes. Centrospheres with conspicuous 

 radiations but without true centrosomes were described in the mitoses of 

 the germinating spore of Pellia by Farmer and Reeves (1894), Davis 

 (1901), and Chamberlain (1903). Gregoire and Berghs (1904), however, 

 pointed out that the centrospheres observed by the foregoing writers in 

 Pellia are in reality only appearances due to the intersection of numerous 

 astral rays, and are not distinct bodies. 





* 1 ' ' "' : ' - s» 



Fig. 23. — Centrosomes in Preissia quadrata. 



A, in fertilized egg just prior to nuclear fusion, B, in cells of young embryo. (After 

 Graham, 1918.) 



In the cells of Preissia quadrata Miss Graham (1918) has more recently 

 made some observations of much interest. She describes and figures two 

 distinct centrosomes with a few astral rays in the cytoplasm of the fer- 

 tilized egg, at the time when the sexual nuclei are approaching each other 

 and in contact (Fig. 23, A). This, together with Yamanouchi's observa- 

 tion on Fucus and that of Williams on Dictyota, cited above, suggests 

 that in certain plants, as in animals, the formation of centrosomes and 

 asters in the egg cytoplasm is in some way induced by the entrance of the 

 sperm. Similar appearances have been noted by Meyer (1911) in Cor- 

 sinia and by Florin (1918) in Riccardia (Aneura). Centrosomes were 

 also observed by Miss Graham in the four-celled embryo of Preissia 

 (Fig. 23, B), this being one of the only cases in which centrosomes have 

 been seen in non-spermatogenous cells in plants above the algae. 



Conclusion. — With regard to centrosomes in plants, it may be con- 

 cluded from the above review that although there is no adequate evidence 

 for their existence in the cells of angiosperms, they are clearly present in 

 many algae, fungi, and probably certain bryophytes. where they perform 



