rilE PROTOPLASMIC SYSTEM 



nucleus) radiations from each polar end of a group of fibre- 

 like structures, the so-called mitotic spindle. (Fig. 3.) 

 The radiations center at a minute granule or at a larger 

 vacuolar sphere-structure; in some cases both granule and 

 sphere are present. The minute body is the centriole or cen- 

 trosome and the sphere is the centrosphere; the radiations 

 because of their disposition are called astral rays. Best 

 seen in cells that possess granules, the rays are also demon- 

 strable in cells which are granule-free. Further, cells which 

 show no rays are often filled with granules. Therefore the 

 rays do not depend upon the cell-inclusions. Seen in the 

 living egg of Platynwreis, for example, every astral radiation 

 is a granule-free track whose greater width is about 3 /i. 

 In the living Arbacia or Echinarachims egg the rays have 

 the same configuration. Nothing in the living or properly 

 fixed egg appears as the stiff coarse "astral fibres" of poorly 

 fixed eggs. The radiations are without doubt no fibres 

 at all. It is often held that they are paths of flow and in 

 living granule-rich cells they do certainly appear to be such 

 — the flow causing a displacement and close packing of 

 granules and thereby accentuating the radiate appearance. 

 There is no unanimity of opinion concerning the nature 

 either of the radiations or of the spindle-" threads" on 

 which the chromosomes move. I share with others the 

 idea that spindle-" threads" and astral radiations are 

 both extremely fine delicately thin sheaths enclosing 

 fluid material, that they are not in any sense fibres with 

 contractile power. (See Fig. zgb and Fig. 33^.) 



As an almost perfectly homogeneous and fluid system, 

 cytoplasm could scarcely show differentiated pathways of 

 flow even in extremely thin-walled tubes; less could it show 

 contractile fibres. Either then cytoplasm is not structure- 

 less or it has power to elaborate structure sufficient to 

 account for the ray-configuration of the astral system. In 

 either case the cytoplasm is diflferent before and after the 



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