JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 49 
those in quadrant D. Figure 61 (Plate 7) shows the anterior surface of 
the egg at the end of the sixth cleavage, and a diagram of a single 
quadrant at this stage was given on page 41. Comparing either the 
figure or the diagram with the scheme (Diagram III.) given on page 46 
for the quadrant D at the same stage, the arrangement is seen to be the 
same except in two respects, (1) The cells belonging to the same lat- 
eral series are equal in A, B, and C, unequal in D. (2) Four ventral 
ectodermal cells additional are present in each of the quadrants A, B, 
and C; these are represented in D by the cells which have passed 
inward to form the entoderm. 
As shown by the spindles in Figure 61, the first cells to divide are 
a3 718 and ato, Phe cleavage is meridional and equal. The 
resulting cells are a% — 8%, 9826. 68.26, a7 — (897. and d. c. They 
are shown in Figures 72 and 75 (Plate 9). (Compare Diagram V.) 
Next follows the division of the six cells, a^9— 6^9 and a^5— c^, which 
together form part of a transverse girdle, surrounding the egg. The 
S4. ale, ar 
cleavage is meridional and equal. The resulting 12 cells, « 
08.12, 815 — C8. 46, and 9919 095 forming as before a transverse girdle, are 
shown in Plate 8, Figs. 69 and 70, and Plate 9, Fig. 71. 
Next ensues the cleavage of the transverse row containing the six 
cells, ac"? and o*7—c7. These cells, as shown in Figure 61 (Plate 7), 
are much flattened dorso-ventrally, and are of exactly the same form as 
the cells last discussed, which lie immediately dorsad of them. Moreover, 
each cell in this row corresponds in origin to a cell of the row last 
described, the two rows having been derived from the equatorial division 
of a previously existing transverse row, as shown in Figure 55. Ik 
mechanieal conditions are decisive in determining the,direction of the 
cleavage, these two rows should cleave in the same manner, i. e. both 
meridionally. Nevertheless, as shown in Figure 69 (Plate 8), while the 
dorsal of the two rows divides meridionally, the cells of this ventral row 
all cleave equatorially. The axis of the cell in which the spindle lies is 
about half as long as the axis which is at right angles to it. The cells 
elongate in the direetion of the spindles, and a very unequal division 
ensues, The ventral products are minute, while the dorsal ones are 
nearly equal in size to the mother cells. Figure 75 (Plate 9) shows 
the anterior surface of the egg after this cleavage. The twelve cells 
18.10 8.10 8.13 8.13 8.14 8.14 
2 , . 
produced are a=, 5.10 — 80. a or Mand us te 
The division of the band of twelve small cells composed of a"? — e, 
a — .o, 911 gr and a.t? = 012, and shown in Figure 61 (Plate 7), 
follows somewhat later. "The cleavage is equatorial and the spindles lie 
VOL. XXX. — NO. 1. 4 
