62 



BIOLOGICAL LECTURES. 



centrosome at each pole, almost in contact with the nuclear 

 membrane, then a group of centrosome granules imbedded in 

 a ground substance ; then comes a clearing of the centre, accom- 

 panied by peripheral arrangement and subsequent fusion of all 

 the centrosome granules, excepting one, which remains in the 

 centre. Thus is established a hollow sphere (" centrosome ") 

 with an included centrosome ("centriole") (Fig. i8). During 

 the early stages of the metamorphosis of the centrosomes, 

 chromatin granules from the nucleus are found in their im- 

 mediate neighborhood, and, as the spindle forms, they become 



closely pressed to the 

 centrosomes (Fig. 17). 

 The entire spindle then 

 moves directly along the 

 prolongation of its axis, 

 thus parallel to the di- 

 rection of elongation of 

 the sphere-substance, 

 to one side of the Q^g 

 (Fig. 18), until the cen- 

 trosome of one end 

 comes almost in con- 

 tact with the peripheral 

 layer of protoplasm. 

 Then comes the m eta- 

 phase (Fig. 18), and, in 

 a late stage of the anaphase, the sphere begins to enlarge, 

 and a reticulum (or vesicular substance) with nodal micro- 

 somes (Fig. 19) develops in it in place of the single centro- 

 some. 



The Qgg now begins to elongate at right angles to the plane 

 of division, and the entire spindle, including the enlarged 

 spheres, shifts towards the centre of the Qg^ a short distance, 

 and finally comes to rest (Fig. 19). The spindle acts as though 

 oscillating through a point of equilibrium, at first shifting too 

 far in one direction (Fig. 18), then swinging back (Fig. 19), and 

 possibly undergoing lesser shiftings before coming to rest for 

 the first cleavage. 



Fig. 18. — Migration of the spindle to one side of the egg ; 

 metaphase ; for the sake of clearness only a few of the 

 chromosomes were drawn in. 



