ORGANIZATION AND CELL-LINEAGE OE 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. The reverse is the case if the egg is 

 viewed from the posterior pole, i.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 41 ) are relatively far apart, those in the anterior-animal cells (a 4 ' 2 ) (dose 

 together (figs. 106 and 109); whereas the reverse is the case in the posterior cells, 

 i.e., the nuclei in the posterior-vegetal cells (B 41 ) are near together, those in the 

 posterior-animal cells (b 4-8 ) far apart (tig. 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 tact 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 seq.). 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 " cross-furrow " on the right and left sides between the anterior dorsal 

 and the posterior ventral cells (A 4 - 1 and b 4 '", figs. SI, 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, 

 123-126, 128, 130, 134, et seq.). I observed the process of formation of this cross- 

 furrow r 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 perjiendicular to 

 the egg axis, as indicated by the faint line between the cells B 41 and b 4 8 of figure 31, 

 which line represents the position of the furrow between those cells when it first 

 appears. A 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 egg 

 hetween 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. 



