ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 45 
anterior pole it is seen that the spindles in the anterior quadrants are not parallel, 
but that they converge toward the animal pole. Тһе reverse is the case if the egg is 
viewed from the posterior pole, z. e., the spindles in the posterior quadrants diverge 
toward the animal pole. Thus it comes about that the nuclei in the anterior-vege- 
tal cells (A*!) are relatively far apart, those in the anterior-animal cells (at?) close 
together (figs. 106 and 109); whereas the reverse is the case in the posterior cells, 
2. е., the nuclei in the posterior-vegetal cells (B*') are near together, those in the 
posterior-animal cells (b**) far apart (fig. 107). 
Every one of these matters is of prospective significance in the further devel- 
opment of the embryo; associated with the forward slant of the spindles toward 
the animal pole is the fact that the cells of the animal hemisphere overhang those 
of the vegetal hemisphere at the anterior pole; whereas the posterior cells of the 
vegetal hemisphere are not completely covered by those of the animal hemi- 
sphere when the egg is viewed exactly from the animal pole (figs. 110, 112, 116). 
Associated with the convergence of the spindles in the anterior quadrants toward 
the animal pole and the convergence of the spindles of the posterior quadrants 
toward the vegetal pole is the fact that in later stages the anterior half of the 
vegetal hemisphere is broad from side to side, its posterior half narrow, while the 
anterior half of the animal hemisphere is narrow from left to right, its posterior 
half broad (figs. 109-118, et seg.). While the position of these spindles is therefore 
indicative of important prospective characteristics of the embryo, it must not be 
regarded as the initial cause of these characteristics. Indications of these features 
may be seen in the distribution of the yolk and protoplasm at the four-cell stage, 
and there can be no doubt that the position of the spindles is itself the result of 
cytoplasmic localization. 
One of the features of this stage to which Castle calls particular attention is the 
presence of a “ eross-furrow " on the right and left sides between the anterior dorsal 
and the posterior ventral cells (A*' and b**, figs. 31, 32, 108, 184). I find, as did 
Castle and Chabry, that this cross-furrow is constant in position and that it marks a 
downward bend in the equator, which may be observed as late as the gastrula stage ; 
in the region of this downward bend the ectoderm cells grow down over the cells of 
the vegetal hemisphere in advance of the neighboring ectoderm cells (figs. 116-119, 
125-126, 128, 130, 154, et seg.). I observed the process of formation of this cross- 
furrow in the living egg, and have represented this in figure 31. When the third . 
cleavage furrows first appear, they are all in nearly the same plane, the furrows 
between the daughter cells of the posterior quadrants being nearly perpendicular to 
the egg axis, as indicated by the faint line between the cells В*! and b“* of figure 31, 
which line represents the position of the furrow between those cells when it first 
appears. А minute or two afterward this furrow is tilted downward at its anterior 
end and upward at its posterior, as indicated by the heavy line between those two 
cells; in this way the cross-furrow arises on the right and left sides of the есе 
between the anterior dorsal and the posterior ventral cells. 
1 Figs. 106 and 107 represent two sections of one and the same egg, in the 8-cell stage, the 
former through the nuclei of the anterior cells, the latter through the nuclei of the posterior ones. 
