260 SOME ASPECTS OF CELL-CHEMISTRY AND CELL-PHYSIOLOGY 



tion-sphere. The fate of such eggs was not determined, but they 

 form a complete demonstration that it is the centrosome and not the 

 nucleus that is the active centre of cell-division in the cell-body. 

 Scarcely less conclusive is the case of dispermic eggs in sea-urchins. 

 In such eggs both sperm-nuclei conjugate with the egg-nucleus, and 

 both sperm-centrosomes divide (Fig. 118). The cleavage-nucleus, 

 therefore, arises by the union of three nuclei and four centrosomes. 

 Such eggs invariably divide at the first cleavage into four equal blas- 

 tomeres, each of which receives one of the centrosomes. The latter 

 must, therefore, be the centres of division. 1 



The statement that the centrosome is an organ for cell-division 

 does not, however, express the whole truth ; for in leucocytes and 

 pigment-cells the astral system formed about it is devoted, as there is 

 good reason to believe, not to cell-division, but to movements of the 



B 



Fig. 117. Eggs of Ascaris with supernumerary centrosome. [BOVERI.] 

 A. First cleavage-spindle above, isolated centrosome below. B. Result of the ensuing division. 



cell-body as a whole ; and, moreover, amitotic division may appar- 

 ently take place independently of the centrosome. The role of the 

 centrosome and attraction-sphere in gland-cells (where they are some- 

 times very large) and in the nerve-cells is still wholly problematical. 

 It would seem, therefore, that the primary function of the centrosome 

 is to organize an astral system, of which it forms the focus, that is 

 primarily an apparatus for mitotic division, but may secondarily 

 become devoted to other functions. The nature of the energy by 

 which this organization takes place is almost wholly in the dark. 

 The extraordinary resemblance of the amphiaster to the lines of 

 force in a magnetic field has impressed many observers, but Roux 

 has proved that the axis of the mitotic figure is not affected, during 

 its formation, by a powerful electro-magnet. The molecules or micro- 



1 This phenomenon was first observed by Ilertwig, and afterwards by Driesch. T have 

 repeatedly observed the internal changes in the living eggs of Toxopneustes. 



