624 GRIFFIN. [Vol. XV. 



groups, which roughly corresponds to that connecting the cen- 

 ters of the copulating germ-nuclei, may be parallel or more or 

 less inclined to the egg-axis. 



PART III. — SUMMARY AND CONCLUSION. 

 Achromatic Structures. 



In respect to gross morphology, the astral systems of Tha- 

 lassema and Zirphaea agree closely with Boveri's description of 

 them in Asca?-is. The minute focal granule may be compared 

 to his "centriole," the cloudy area to his " centrosome," and 

 the centrosphere to the "heller Hof," which with the dense 

 crown of astral rays is equivalent to his "astrosphere." 



A detailed study of all stages in their successive order 

 leaves, however, no possible doubt that in the forms I have 

 studied the minute black focal granule ("centriole") is func- 

 tionally here the true centrosome, as understood by Boveri — 

 the " single permanent cell-organ which forms the dynamic 

 center of the cell and multiplies by division to form the centers 

 of the daughter-cells." It alone of all elements of the astral 

 system persists tlirouglwiit all stages, divides, and apparently 

 initiates mitotic activity. The rays, centrosphere, and cloudy 

 area, which are in turn differentiated about the centrosome, are 

 formed only during the prophases and metaphases. No sooner 

 is the anaphase fairly under way than these begin to break 

 down, while the centrosome, its mission accomplished, migrates 

 to the periphery of the sphere and there sets up a new system 

 often superimposed upon remnants of the old. 



By a careful study of these processes, the impression is most 

 strongly conveyed that throughout all these stages the cetitro- 

 sonie is the cause ratlier than the mere expression or bye-product 

 of the aste7'-for7nation. This is especially clear in late anaphase, 

 where the centrosome deserts the old system and, moving to a 

 different locality, furnishes there the stimulus to the formation 

 of a new one. It is hardly less obvious at the close of the 

 "pause" (during copulation of the germ-nuclei), where after 

 almost complete dying down of the asters the centrosome again 

 furnishes the stimulus to renewed activity. 



