BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
i ge D « 4 
Division of Quartet 5.2, forming 6.3 and 6.4. 
Same Figures as for 5.1. 
Figure 23 shows the quartet at the close of the division which has 
resulted in the formation of the quartets 6.3 and 6.4. The nuclear con- 
ditions in like manner show that this division has also taken place in a 
right spiral. The renmants of the spindles are parallel to those of the 
quartet 5.1, and the upper derivative lies to the right of the lower one. 
These divisions are also approximately equal and synchronous. The 
arrangement of the cells of the vegetative pole is very regular. The ven- 
tral cross furrow remains approximately at right angles to the dorsal 
furrow. The two cells 0% and dê? meet in this ventral furrow, and are 
hexagonal in outline, while the other two members of the quartet un 
and c9! are pentagonal. All four cells of the quartet 6.2 are hexagonal, 
as will be seen in Plate IV. Figs. 27-29, and 31. A comparison of the 
quartet 6.1 of Figure 27 with 5.4 of Figure 28 shows how little differ- 
ence there is in the size of the cells of the two poles in the superficial 
view. In optical section, however, a distinct difference can be detected 
in the size of the deeper lying parts of these cells. In Figure 30 (Plate 
IV.) is represented such a section taken from the egg in the position 
shown in Figure 29. The section passes through the vertical axis in 
the plane corresponding to that of the paper in Figure 29, thus cutting 
the ventral cross furrow at right angles, and passing through the quad- 
rants b and d. It will be seen that one of the two cells of the vegeta- 
tive pole, labelled d* in the drawing, is much larger than the other, po 
This is the cell which at the next generation gives rise to tho first 
mesoderm cell, d^?, or M. 
The generalization which Rabl made in his paper on Planorbis (79), 
— “dass bei den Keimen mit reichlichem Nahrungsdotter von dem Zeit- 
punkte an, als Aequatorialfurchen auftreten, die Zellvermehrung in 
arithmetischer, bei den Keimen mit spárlichem Nahrungsdotter dagegen 
zuerst in arithmetischer, sodann aber in geometrischer Progression 
erfolgt,” — is not borne out by the cleavage in Limax, as tha twelve-cell 
stage does not regularly occur. ‘The cleavage in Limax runs, 
E SO A DR 
on 0:0:0+0:0+0:0+0+0+(0. 
This is fundamentally an arithmetical progression, a series whose common 
difference is four, the mathematical expression of the increase in the 
number of cells in the spiral type of cleavage. It varies, however, from 
