72 



CELL-DI VISION 



facts in its support, though neither his observations nor those of later 

 investigators have sustained Van Beneden's account of the grouping 

 of the astral rays. Boveri showed in the clearest manner that, during 

 the fertilization of Ascaris, the astral rays become attached to the 

 chromosomes of the germ-nuclei ; that each comes into connection with 

 rays from both the asters ; that the chromosomes, at first irregularly 

 scattered in the Q,g^, are drawn into a position of equilibrium in the 

 equator of the spindle by the shortening of these rays (Figs. 65, 104); 

 and that tJie rays thicken as tJicy sJiortcn. He showed that as the 

 chromosome splits, each half is connected only with rays (spindle- 

 fibres) from the aster on its own side; and he followed, step by step, 



B 



/ 



Fig. 35. — Leucocytes or wandering-cells of the salamander. [Heidenhain.] 

 A. Cell with a single nucleus containing a very coarse network of chromatin and two nucleoli 

 (plasmosomes) ; s. permanent aster, its centre occupied by a double centrosome surrounded by 

 an attraction-sphere. B. Similar cell, with double nucleus; the smaller dark masses in the latter 

 are oxychromatin-granules (linin), the larger masses are basichromatin (chromatin proper). 



the shortening and thickening of these rays as the daughter-chromo- 

 somes diverge. In all these operations the behaviour of the rays is 

 precisely like that of muscle-fibres ; and it is difficult to study Boveri's 

 beautiful figures and clear descriptions without sharing his conviction 

 that "of the contractility of the fibrillae there can be no doubt." ^ 



Very convincing evidence in the same direction is afforded by 

 pigment-cells and leucocytes or wandering-cells, in both of which 

 there is a very large permanent aster (attraction-sphere) even in the 

 resting-cell. The structure of the aster in the leucocyte, where it 

 was first discovered by Flemming in 1891, has been studied very 



1 '88, 2, p. 99. 



