KOFOID: DEVELOPMENT OF LIMAX, 53 
in Neritina (Blochmann '81) are similar to those of Umbrella, The con- 
ditions in the eight-cell stage of Planorbis (Rabl '79) are complicated by 
the fact that this pulmonate probably has reversed cleavage (cf. Rabl ’79, 
Taf. XXXII. Figs. 7, 9), and that therefore the mesoderm arises by a 
right spiral instead of a left one, as in the case of the unreversed or 
normal type of cleavage. Orienting the Planorbis egg for the purpose 
of comparison after the method employed by Wilson (92) for Nereis, 
we have the first mesoderm cell arising from quadrant C' instead of 
D; and in the four-cell and later stages the ventral cross furrow lies 
between A and C instead of between B and D (cf. Rabl '79, Taf. 
XXXII. Figs. 7, 8B, 11 B). The differences between Limax and 
Planorbis will be best shown by a comparison of Figures A and B with 
the corresponding stages of Planorbis given in the diagrams below. 
Ficunz C. Figure D. 
Figure C is a diagrammatic representation of the four-cell stage of 
Planorbis, showing dorsal and ventral eross furrows parallel. Combined 
from Rabl’s (79) Taf. XXXII. Figs. 7-12. Figure D, the same of tho 
eight-cell stage. 
It will be seen that in Planorbis, as in Umbrella and Neritina, — all 
of them forms with considerable yolk, — the dorsal and ventral furrows 
are both formed by the contact of cells of the same quadrants. On the 
other hand, in Nereis and Limax the furrows of the two poles are 
formed by the contact of cells of diferent quadrants. In Nereis, how- 
ever, the dorsal furrow is comparatively shorter than in Limax, and in 
Umbrella it is of still less extent. 
I have observed no difference in tho time of cleavage of the different 
quadrants. The nuclear conditions in Figure 19 (Plate IT.) indicate 
that the division is very slightly more advanced in the posterior half of 
