CH. iv] THE CENTROSOME 43 



of the old spindle, and a fresh nuclear division commences. 

 But, as will be seen shortly, the centrosome frequently 

 undergoes considerable changes during the progress of a 

 nuclear division, and in these cases it is not certain that 

 the centrosome of one nuclear division is identical with that 

 of the preceding, though it may be formed from it. The 

 opinion, therefore, that the centrosome is a permanent 

 cell-organ, as constant as the nucleus, is of very doubtful 

 validity, and in cells which are not dividing often no trace 

 of such an organ may be found. 



In some cells, however, the centrosome certainly persists 

 as a permanent organ, for in certain flagellate cells, such as 

 the collar-cells of sponges and the bodies of some of the 

 Flagellate Protozoa, the centrosome is the basal granule of 

 the flagellum, and the same is true of the mother-cells of 

 spermatozoa, in which the axial filament of the "tail" 

 grows out from the centrosome. In many epithelial cells 

 also the centrosome can constantly be seen as a single or 

 double granule. 



Those who hold that the centrosome is in some sense a 

 permanent organ regularly existing in every cell regard it 

 as arising originally from the "middle piece" of the sperma- 

 tozoon which fertilised the egg from which the animal 

 originated. There is no doubt that the middle piece of the 

 spermatozoon is at least partly formed from the centrosome 

 of the cell which gives rise to the spermatozoon, and that 

 after fertilisation the centrosomes of the first segmentation 

 division of the egg arise either from or in connection with 

 the middle piece (see Chapter VIII). But as will be seen 

 below there is no certainty that the centrosomes of the first 

 division are actually produced by division of the middle 

 piece, and it seems equally possible that they originate from 

 the egg-cytoplasm in consequence of some chemical change 



