iv] ORIGIN OF CENTROSOME 47 



merge altogether in the spheres, but in anaphase a new 

 small centrosome containing a central granule appears in 

 each sphere, and in the telophase the granule or centriole 

 divides in preparation for forming the centrosomes of the 

 next succeeding division. 



JENKINSON interprets these observations thus. Themiddle 

 piece of the spermatozoon dissolves producing a centre of 

 high osmotic tension to which water is drawn from the sur- 

 rounding cytoplasm. This gives rise to the vacuolation and 

 outgrowth of radiating alveoli, and the path of the water 

 travelling from every side towards this centre causes the 

 outer radiations. Then nucleinic acid from the head of the 

 spermatozoon (sperm-nucleus) causes a precipitation within 

 the vacuolated sphere, and this is the definitive centrosome. 

 In the later stages of division a somewhat similar process 

 recurs, giving rise in the telophase to the centrosomes 

 which will act as poles of the spindle in the next division. 

 The bearing of these observations on the formation of the 

 spindle itself will be considered later; here it may be 

 noticed that the formation of a centrosome by precipitation 

 at the centre of a system of rays or alveoli is just what has 

 been described as taking place in unfertilised eggs of various 

 sorts under the influence of chemical reagents, and if 

 JENKINSON'S observations may be accepted as typical, 

 they seem to provide a middle way between the two 

 divergent views that centrosomes always arise from 

 pre-existing centrosomes, or that they may be formed de 

 novo in the cytoplasm. In corroboration of JENKINSON'S 

 interpretation of the origin of the centrosome it should be 

 mentioned that according to F. R. LILLIE (1919) the 

 middle piece of the spermatozoon of Nereis is thrown off 

 with the tail and the aster and centrosome arise in con- 

 nection with the nuclear substance of the head. 



