THE MITOTIC CYCLE 



produced by a variety of experimental treatments, though neither 

 Gray^"^ nor Wilson^^^ in reviewing this work is convinced that the 

 possibihty has been wholly excluded that material from nucleus or 

 centrosome may have contributed to the accessory asters. 



Some have doubted whether the centriole is more than a fixation 

 artefact. In Fischer's^"^ monograph on the action of fixatives and 

 stains on biological material, their effect on the achromatic figure is 

 one of the main subjects of inquiry. The author reviews previous 

 descriptions of the centriole, and concludes : 'Alle diese Beschreibungen 

 reichen nicht aus, um durch die Gestalt granulationen zu unterscheiden 

 oder von etwaigen Fallungsartefacten'. About 30 years later, Fry re- 

 examined this question by studying the effect of various fixatives on the 

 appearance of the aster and its central region, mainly in marine eggs; 

 in the first of this series, cytasters induced in activated Echinarachnius 

 eggs were examined. The texture of the asters in the stained sections 

 varies with the fixative used, and it was found that 'central bodies are 

 present only when the fixative coagulates the cytaster in such a manner 

 that rays are distinctly fixed and extend to the centre'. In succeeding 

 papers (Fry^"^ ^^^) this conclusion was extended to the sperm aster and 

 first cleavage figure of the Echinarachnius tg^, and to the first metaphase 

 spindle of Chaetopterus (Fry^^^) . 



Fry did not attempt to generalize from these results, and in the 

 last paper suggests a classification of central bodies as seen in fixed 

 preparations in which his 'focal staining artefacts' are but one category, 

 clearly distinguishable from true centrioles. These are 'stable structures, 

 which unlike focal bodies, maintain their characteristic size and shape 

 in spite of extensive modifications in the coarseness and shape of the 

 rays and fibers'. Fry's work evoked a vigorous reaction from several 

 cytologists, who cited a number of instances where centrioles can be 

 traced as permanent cell organs. Some of the classical examples were 

 re-investigated. Pollister^i^ maintained that in the amphibian leuco- 

 cyte, the centriole was not a coagulated focal point, because in inter- 

 phase it is a double structure eccentrically placed inside a single aster; 

 a similar argument had been used by Boveri^'*^ in answer to Fischer. 

 HuETTNER^^^ was able to trace the continuity of the centrioles during 

 the cleavage of the tgg oi Drosophila melanogaster, and in a further paper, 

 HuETTNER and Rabinowitz^i* describe central bodies in the pole cells 

 of the living egg and their movement round the surface of the nucleus 

 in prophase. 



Other authors have also identified centrioles in living cells. Those of 

 the hypermastigote flagellates which have been described by Cleve- 

 LAND^is 216 a^j-e j_2jj, in width, and up to 8o[x in length; from them the 

 origin of the achromatic figure and of all other extranuclear organelles 

 has been clearly traced. Johnson^^^ has followed them through the cycle 



108 



