240 LILLIE. [Vol. XVII. 



becomes stretched out to form the periphery of the central 

 spindle (PI. XXV, Fig. 17), and the radiations entirely dis- 

 appear, leaving the spindle in the center of a mass of vesicu- 

 lar cytoplasm. The outer end of the spindle pushes up through 

 the group of chromosomes nearly to the periphery of the Q.gg. 

 In Fio-. 17 the first faint mantle-fibers can also be seen stretch- 

 ino- from each chromosome to the inner centrosome. 



Fig. 18, PI. XXV, illustrates a later important phase in the 

 elongation of the spindle. One of the most interesting fea- 

 tures of this stage is the change which the inner centrosome 

 has already undergone, to be described in detail immediately 

 below. The elongation of the spindle, after the outer end has 

 become fixed at the periphery of the Qgg, has resulted in the 

 passive drawing out of the protoplasmic vesicles in straight 

 rows extending to the surface. The appearance of crossing of 

 rays seen in this and other figures is due, I am convinced, to a 

 double strain on the same mass of protoplasm. The chromo- 

 somes have now been drawn, apparently by their attached 

 fibers, around the equator of the spindle. The asters are 

 beginning to develop at both ends by continuous arrangement 

 of adjacent alveolar walls. This is very plainly shown by com- 

 parison of Figs. 17 and 18, PI. XXV. 



Figs. 19, 20, and 21, PL XXV, illustrate successive later 

 stages of the same processes and need not be described in 

 detail here. They carry us to the metaphase of the second 

 maturation spindle. The structure of the centrosome, spheres, 

 and aster in Fig. 20, PI. XXV, is precisely the same as in 

 the stage with which we started (PI. XXIV, Figs. 9 and 10). 

 Indeed, at the metaphase of any spindle we have the same 

 structures. // is always at the metaphase that the eentrosotne is 

 smallest ; at this stage the inner ajid outer spheres are also 

 invariably marked by concentric rows of m,icrosomes. 



Now, how are these structures related to the stage of 

 Fig. 15, PI. XXV, with which we ceased our description of 

 centrosome structure.'' In this stage the large centrosome has 

 simply increased in size and at the same time become more 

 subdivided. Figs. 24-27, PI. XXV, illustrate the changes 

 which the centrosome undergoes in assuming the form charac- 



