44 CENTROSOMES, SPINDLE AND ASTERS [CH. 



induced by the middle piece of the spermatozoon, or pos- 

 sibly in some cases by the nuclear substance of its head. 

 That the egg-centrosomes do not always have such a source 

 is proved by their presence in the segmentation divisions of 

 parthenogenetic eggs (eggs which develop unfertilised). Some 

 observers have maintained that in this case the centrosomes 

 arise from those of the last (maturation) division of the egg- 

 nucleus, but others have described the origin of centrosomes 

 de novo in non-nucleated fragments of eggs under the influ- 

 ence of various chemical reagents, and at the present time, 

 although the question is by no means completely settled, 

 the evidence tends to show that centrosomes can arise 

 afresh from the cytoplasm, and that they are temporary 

 structures produced by some chemical change concomitant 

 with nuclear division, and are not in any true sense invari- 

 able constituents of the cell. This conclusion is supported 

 by the fact that true centrosomes appear to be absent in the 

 higher plants, in which the process of mitotic division is 

 otherwise similar to that in animals. 



Considerable confusion has been introduced into the 

 literature of the subject by the rather different senses in 

 which the word centrosome has been used. At an early 

 stage, before the spindle is completely formed, the centro- 

 some usually has the form of a small deeply staining granule 

 surrounded by a rather dense mass of protoplasm. Some 

 investigators use the word centrosome for the whole 

 structure, and give the uamGcentriole for the central granule ; 

 others call the granule a centrosome, and speak of the 

 surrounding mass as the cmtrosphere (or astrosphere) 1 . It 



1 Strictly, the word astrosphere was originally used for the structure 

 surrounding the centrosome, and centrosphere for the whole body centrosome 

 and astrosphere. The older term "attraction-sphere" is equivalent to 

 astrosphere. 



