No. 2.] CENTROSOMES IN THE ANNELID EGG. 195 
But although the structure of the reticulum, the peripheral 
egg membrane (pellicle), and the nuclear membrane are easily 
demonstrable and beautifully clear throughout the cytoplasm, 
there is, as yet, 2o trace of anything suggesting a centrosome or 
an aster. As the egg grows larger, however, the outlines 
of the meshes become polygonal rather than circular (Fig. 6), 
and show rather pronounced nodes. Eggs which have reached 
this stage of development, when placed in sea-water continue 
to develop as far as the formation of the first maturation-spindle. 
The tendency of the fibrils of the network to straighten becomes 
accentuated, so that many of them extend in straight lines for 
a distance several times the diameter of the single meshes 
(Fig. 6). Moreover, these longer fibrils radiate from common 
centres, and in this way there arise in the cytoplasm a number 
of miniature asters (Figs. 6,7). At first the asters possess only 
two or three rays, but the latter soon increase in number and 
in length at the direct expense of the remaining network. The 
formation of asters continues until a climax is reached, when 
one can count no less than seventy-five distinct asters scattered 
about through the cytoplasm of a single egg. They are most 
numerous in the zone formerly occupied by the paranucleus. 
These structures correspond closely to the “ secondary mechani- 
cal centres” of Reinke, and, for reasons which appear further 
on, I have called them secondary asters) 
All stages in the development of the asters out of the polyg- 
onal network may be represented in a single section, yet often 
many of the larger asters are approximately equal in size, 
and, though distinct from one another, are frequently so 
close together that their rays intercross (Fig. 7). The nuclear 
membrane now presents a peculiar appearance, being drawn 
out into numerous sharp points—a phenomenon which is 
probably correlated with the development of the multiple 
asters (Fig. 7). 
The period of development characterized by the multiple 
asters is not of long duration. Two of the asters gain pre- 
dominance over the others in point of size, and continue to 
grow larger, while the others gradually evanesce (Fig. 8). 
1 Mead ('97a). 
