The Development of Ischnochiton. Q13 



quadrants are the first to show signs of division. However, their 

 acceleration is at first not of much consequence as a glance at Fig. 20 

 will show. The next division is at right angles to the first and 

 affects all the cells produced by the above division (Figs. 26, 27). As 

 in the preceding case the result is an equal cleavage in each cell 

 making a quadrangular group of four cells in each quadrant. The 

 acceleration of cleavages is here more marked (Fig. 27). 



The next cleavage occurs in the upper cells in each group 

 (Fig. 30) and is later followed by the lower products (5 a-?- ^••2, Sa^-^-^ 

 etc.). The spindles of both these cleavages are horizontal and are 

 at right angles to the preceding. In this case each cell cleaves into 

 equal products, and the division is usually completed in the posterior 

 cells before it has advanced to the stage of the metaphase in the 

 anterior quadrants. 



The time difference between the anterior and posterior quadrants 

 soon becomes very marked and an irregularity in the direction of the 

 anterior cleavages sooner or later creeps in, so that they become a 

 confused irregular group of cells occupying the space right and left 

 of the mid line between the second quartette, blastopore and proto- 

 troch (Fig. 47). The cleavages do not cease but the cell growth is 

 apparently small for while the cells become more numerous they do 

 not expose a greater surface as a result. This continues as far as 

 the group can be followed, which represents a stage when the blasto- 

 pore has shifted nearly to the prototroch, and it appears probable 

 that it never occupies a much greater area than it did early in its 

 history. 



In the posterior quadrants however matters are different. In the 

 upper dorsal portions of each of the two groups the cells continue 

 to possess a great regularity, exhibiting the appearance of stones in 

 a wall (Fig. 46, 47), but in those portions anterior to the growth zone 

 and forming the ventral surface of the embryo the cell divisions early 

 become irregular. 



The growth zone which becomes established before the blasto- 

 pore has shifted to any great extent appears to arise in the neigh- 

 borhood of the prototroch and gradually work downward. It has 

 already been noticed that the upper cells of the third quartette divide 

 before the lower (Fig. 36), and Fig. 44 shows a series of cleavages 

 in the row next to the top while in Fig. 46 the most rapid cell 

 divisions have occurred in the fifth row. At a slight distance below 

 this point the growth zone becomes located permanently (Fig. 49) 



