20 ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 
the surface of the egg and the chorion, are sometimes carried down with the stream- 
ing protoplasm to the iower pole of the egg, where they are crowded together and ` 
heaped up in the perivitelline space (fig. 3, e¢ seg.). While this flowing is most 
active, streamers of yellow surface protoplasm may be seen radiating toward the 
upper pole. The yellow protoplasm thus carried to the lower pole collects into a 
deep orange-yellow spot which surrounds the sperm nucleus (figs. 4-6); it frequently 
forms a prominence at the lower pole which recalls the polar lobe of the eggs of 
annelids and mollusks. The clear nuclear protoplasm also flows to the lower pole, 
where it lies beneath the yellow disk or spot and is visible around its periphery (figs. 
4-6). The yellow protoplasm then gradually spreads again until it covers most of 
the lower hemisphere (figs. 6-10). Then the sperm nucleus moves to one side of this 
yellow cap, and a large part of the yellow protoplasm is drawn over with it until it 
forms a yellow band or crescent, in the middle of which the sperm nucleus lies. 
This crescent lies just below the equator of the egg and its middle point marks 
the posterior pole of the future embyro, while its two horns reach forward about 
half-way around the egg to the middle of the right and left sides. | 
b. Localization of Clear Protoplasm and Yolk. 
At the same time that the yellow protoplasm is being formed into a crescent 
and moved up toward the equator on the posterior side of the egg, the clear proto- 
plasm which surrounds the sperm nucleus and aster is also drawn entirely away 
from the lower pole to the posterior side of the egg and thence up to the equator 
(figs. 82-92). Up to this time the sperm nucleus and the clear and yellow proto- 
plasm have remained near to the egg surface; finally, after the meeting of the germ 
nuclei near the posterior pole of the egg, thése nuclei and the clear protoplasm sur- 
rounding them move inward to the center of the egg, while the yellow protoplasm 
is largely left at the surface. 
When the clear and yellow protoplasm are withdrawn from the upper pole the 
gray yolk is there exposed (figs. 4, 5, 11). After the protoplasm moves up to the 
posterior pole the yolk is exposed over the entire egg, except for the area of the 
yellow crescent and a narrow line of clear protoplasm, which comes to the surface 
just above the crescent (figs. 15-18). 
. In sections, small spherules which probably represent the yellow granules 
of the peripheral layer of protoplasm, may be seen heaped up around the entering 
sperm (fig. 74), this aggregation corresponding to the yellow spot of the living egg 
(fig. 6). This massing of the yellow spherules is most marked, while the sperm 
head lies in the peripheral layer; when it passes through this layer into the deeper 
layer of clear protoplasm the yellow spherules again spread out into a flattened disk, 
as shown in figures 75 and 80, which correspond to figures 7 and 8 of the living 
egg (Plate I). Later, when the sperm nucleus moves to the posterior pole and the 
зеке protoplasm is drawn over to that side to form the crescent, sections show 
that this crescent does not lie entirely on the surface, but that it extends for some 
distance inward toward the sperm nucleus (figs. 87, 90, 92). 
