44 ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 
stages the yolk ceases to be peripheral in position and comes to lie in the central 
portions of the cleavage cells (text figs. XVII-XXIV); this change in position is 
brought about by the flowing of the peripheral layer of yolk inward along all the . 
developing cleavage furrows (figs. 104—107, et seg.) until finally the yolk comes to 
occupy a central position in all the blastomeres, while the clear protoplasm 18 
brought once more to the surface. These cytokinetic movements which accompany 
cell division do not change the relative distribution of yolk and cytoplasm in the 
different hemispheres and quadrants of the egg, but only its location in the indi- 
vidual cleavage cells. 
In the second cleavage the constriction of the cell begins at the periphery or 
free surface and proceeds inward through the cell body (figs. 104, 105). The 
peripheral layer of yolk is thus carried inward along the cleavage furrow, as has 
been said, and the middle of each spindle is bent in toward the center of the egg 
(fig. 105). At the same time the yolk and yellow protoplasm continue to be car- 
ried in along the first cleavage furrow. The inflow of yolk along a developing 
cleavage furrow leaves a protoplasmic connection between the two daughter cells, 
into which the yolk does not penetrate for a considerable time; this protoplasmic 
connection is frequently of service in determining the lineage of cells since it always 
connects daughter cells (figs. 104-107). Finally the inflow of yolk completely cuts 
off this connection. 
The four cells which are formed by the second cleavage are all approximately 
of the same size in Czoza ; in Cynthza the two anterior cells are slightly larger than 
the posterior ones, just as Van Beneden and Julin found to be the case in Clavel- 
lina (text figs. ҮП, VIII). But though the daughter cells are of nearly the same 
size they are of very different quality. The posterior cells contain about the same 
quantity of clear protoplasm as the anterior ones, but they contain little yolk and 
practically all of the yellow crescent substance; the anterior cells on the other hand 
. contain a great deal of yolk, but practically none of the crescent substance. The 
second cleavage is therefore differential in a very marked degree (cf. figs. 29 and 30). 
9. Third Cleavage; 4-8 cells. (Figs. 31-55, 106-109, 184). 
In the anaphase of the second cleavage the centrosomes elongate in the verti- 
cal axis and the daughter centrosomes, moving to the upper and lower poles of the 
nuclei, form the centrosomes of the third cleavage spindles. In an abnormal egg 
. shown in figure 103 this division of the centrosomes occurs in one of the blastomeres 
in the prophase of the second cleavage and not in a vertical but in a horizontal direc- 
tion. The position of these third cleavage spindles is peculiar and of great prospective 
significance. They are slightly eccentric toward the animal pole, and accordingly 
the four cells which are cut off at this pole are smaller than those at the vegetal 
pole. When the egg is viewed from either the right or the left side the spindles in 
the anterior and posterior quadrants seem to be parallel and are both slanted for- 
ward at the upper pole; accordingly the four upper cells, when formed, lie slightly 
anterior to the four lower ones (figs. 108, 184). When the egg is viewed from the 
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