40 ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 
exactly in the center of the egg (fig. 96), always being oriented so that it lies at 
right angles to the plane of the first cleavage and with its equator in that plane. 
At the same time the clear protoplasm and a small portion of the yellow protoplasm 
move inward from the posterior pole toward the center of the egg (figs. 92, 96). 
The larger part of the yellow protoplasm remains at the surface in the form of a 
crescent, but the clear protoplasm is entirely withdrawn from the surface except for 
a narrow zone which lies just above (ventral to) the crescent on the posterior side 
(figs. 13-18, 96). During the formation of the first cleavage furrow, even this 
narrow zone of clear protoplasm is withdrawn from the surface to the center of the 
egg, so that the yolk now covers the entire surface of the egg except for the area 
of the crescent (figs. 100—102 and 178-179). "This condition is just the reverse of 
that which prevailed at the beginning of development, when the yolk was central 
in position and the protoplasm peripheral (figs. 76—79). 
The centrosomes and asters are larger and more easily studied in Czoza than in 
Cynthza. Proceeding from the periphery to the center, the following parts of the aster 
may be recognized (figs. 177, 179) : (1) The deeply staining, peripheral layer of the 
aster, (2) the clear inner layer of the aster traversed by radiating fibres, (3) a granular 
central body upon which the astral fibres end. Тһе latter is the centrosome, and 
is plainly composed of two parts, (а) an outer granular zone and (4) a central clear . 
area from which the netrum arises. In Cyzs/Aza the outer and inner layers of the 
aster are not distinguishable and the centrosome itself is not so large as in (ола; 
the latter is, however, composed of the same parts, vzz., a peripheral granular zone 
and a central clear area which gives rise to the netrum (figs. 98, 99). In these 
ascidians, just as in the gasteropods which I have studied (Conklin, 1902), the 
centrosome undergoes a decided growth and metamorphosis during the cycle of 
division; in the early stages of the cycle it is a small, deeply staining body, in the 
later stages it becomes much larger and differentiates into the outer granular zone 
and the central clear area (cf. figs. 97, 98). 
In Czoza both the first and second cleavage spindles are remarkable in that at 
all stages of the division the nuclear part of the spindle can be clearly distinguished 
from the polar or astral part (figs. 177, 179). "The portion of the spindle derived 
from the linin of the germ nuclei is short, deeply staining and barrel-shaped, and in 
all respects resembles a maturation spindle (compare figs. 177 and 179 with figs. 67, 
70, 71). Even in the possession of a few peripheral fibres which radiate from the 
slightly rounded ends of the spindle toward the equator, this spindle resembles 
those of the maturation divisions. "These peripheral fibres are not in line with the 
astral radiations, and hence are all the more striking. Тһе astral rays which run 
from the centrosomes to the ends of this nuclear spindle are small and faintly stain- 
ing as contrasted with the heavy, deeply staining fibres of the nuclear spindle. No- 
where else, so far as I am aware, is this double character of the mitotic figure so 
clearly shown as in these cleavages of Czoza. This is due to the small size of the 
nuclear spindle and to the large size of the astral systems, so that the ends of the 
nuclear spindle are not easily confused with the astral rays, and also to the great 
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