JENNINGS: DEVELOPMENT OF ASPLANCHNA IIERRICKII. 27 
Dorso-ventral Lateral Ratio of Dorso-ventral to 
Cell, Measurement, Measurement, Lateral Measurement. 
atl 25u (2041) 52u 1to 2 (about) (2 to 5!) 
at2 35 50u in 
btl 24u 42u 4“ 7 
i2 35 42 DaN 
one 30% 49 3 “ 56 (about) 
oe 33 65 2 “ 3 (about) 
After the spindles become completely formed, the cells begin to 
elongate in the direction of the spindles. A slightly later stage than 
that just described is shown in Plate’ 4, Figure 26. Comparing this 
with Plate 3, Figure 22, it is evident that the cell % has stretched in 
the direction of the spindle to such an extent that the difference between 
the two axes of the cells is much diminished. Nevertheless, in both this 
cell and a*? the axes in which the spindles lie are distinctly the shorter. 
This is still true at the time of the division of the cells. Figure 27 
(Plate 4) shows the right side of the egg last considered ; in the quad- 
rant C the processes are much more advanced than in A. The nuclei 
have separated and the cytoplasm is dividing, yet exact measurements 
both of surface views and optical sections show that the greater diam- 
eter is still at right angles to the line joining the two nuclei. A frontal 
seotion, showing the greatest dorso-ventral extent of the cells of the 
quadrants A and C of this egg, is given in Fi 
887 
gure 28. 
The two figures last mentioned show another fact of importance. The 
divisions do not separate the blastomeres into cells of equal size in the 
quadrants 4, , and C. The completed cleavage is shown in Plate 4, 
Fig. 30 (anterior view). This, with the figures just cited, shows that the 
cells at? — c*? divide very unequally, the dorsal derivatives, a?*— c**, being 
very much larger than the ventral ones, a$? — 65.8. The inequality is less 
in the division of the ventral cells a*! —c**, Although the ventral de- 
1 1 
rivatives, a? — c*!, occupy a larger area on the surface of the egg, there 
is little difference in actual volume, and such as occurs is in favor of the 
more dorsal cells a5:? — £52 
The order of division is the same as in the last cleavage ; first, the 
quadrant D, then in order C, B, A. In quadrant D tho 
divide at the same time, 
The important facts in this fourth cleavage, from a cyto-mechanical 
standpoint, may be summarized as follows. 
! In every ease the flrst measurement was taken through the two asters; in the 
(ase of at the real dorso-ventral extent of the cell, into which the spindle later 
novcs, is but 205, — so that the ratio is as two to five. 
