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 seq.) until finally the yolk comes to 

 occupy a central position in all the blastomeres, while the clear protoplasm is 

 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 

 oft' this connection. 



The four cells which are formed by the second cleavage are all approximately 

 of the same size in Ciona ; in Cynthia 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. VII, 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 (r/". figs. 29 and 30). 



3. Third Cleavage; 4-8 cells. (Figs. 31-35, 106-100, 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 oft* 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 



