JENNINGS: DEVELOPMENT OF ASPLANCHXA HERRICKII. 47 



three quadrants given on page 41, shows that the directions of the cell 

 walls are the same in both, the inequality in the size of the cell in 

 the quadrant D being the only diflference. 



In the sn'enth cleavage, the spindle appears first in the cell d'^-^, as 

 shown in Figure 58, and the cell is divided ineridionally into two equal 

 cells, a(^*" and d^-'^^. The finished division is shown in Plate 8, Figs. 

 66, 67, and 68. The plane separating these two cells is the median 

 dorso-ventral plane of the embryo, as will be shown later. 



Shortly after this division is completed, spindles appear in d'^-^, d''-^, 

 d'''^", d''-^^, and o?''-", as shown in Figure 66. In d?-^ the spindle is 

 dorso-ventral, hence lying in the shorter axis of the cell ; the cell 

 extends and divides into two equal dorsal and ventral parts, d^-^ and c?^-^° 

 (Figs. 67 and 68). The greater surface extension of d^-^ in Figure 67 

 is due to its being spread out in a thin layer over the surface of the 

 entoderm cell. 



In d'^-^ and cZ''-^° (Fig- 66) the spindles are also dorso-ventral in 

 position, and the cells divide equatorially into equal parts, d^-^"^, d^-^^^ 

 (f«•l^ and d'--° (Figs. 67 and 68). 



In d'-'^^ and d''-^* the spindles lie at right angles to those just described, 

 and the cells divide meridionally and equally, forming d^-"^^, c?^-^®, d^-'^'', 

 and d^--^. 



Figure 67 is a view of this region after these cleavages are finished. 

 As shown in this figure, a certain amount of shifting has taken place 

 during cleavage, by which the cell d''-'' has been excluded from its share 

 in the boundary of the blastopore. As the cells divide, they withdraw 

 their interior pai'ts and extend in the direction of the spindle, as has 

 been minutely described for other cleavages ; in this way the dorso- 

 ventral extent of quadrant D has been greatly increased. As a result 

 the blastopore has been nearly closed (Fig. 63), and at the opposite 

 end the animal pole has been pushed beyond the micromere end of the 

 egg to its anterior side (Fig. 65). 



Thus far the six larger left hand cells have divided, leaving six smaller 

 cells (at the right and at the dorsal pole) undivided (Fig. 67). 



Next, as shown in this figure, the cells d''-'' and c?^-® form spindles and 

 divide. Each cleaves in the same manner as its larger companion cell 

 has done, d''-'' equatorially, d''-^ meridionally. The products, d^-'^^, c?^-", 

 d^-'^^, and c?®-^®, are shown in Figure 68 (compare Diagram lY.). 



Next d''-^^ cleaves equatorially, like its companion cells d''-^ and rf''". 

 In o?''-^^ the division is unequal, the ventral product, d^-^^, being much 

 the smaller (Fig. 68). 



