KOFOID : DEVELOPMENT OF LIMAX". 53 



in Neritina (Blochmann '81) are similar to those of Umbrella. The con- 

 ditions in the eight-cell stage of Planorhis (Rabl '79) are complicated by 

 the fact that this pulmonate probably has reversed cleavage (cf. Eabl '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 

 J) ; 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, 8 B, 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. 



FlGCRE C. FlfiUKE D. 



Figure C is a diagrammatic representation of the four-cell stage of 

 Planorbis, showing dorsal and ventral cross furrows parallel. Combined 

 from Rabl's ('79) Taf. XXXII. Figs. 7-12. Figure D, the same of the 

 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 different 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 the time of cleavage of the different 

 quadrants. The nuclear conditions in Figure 19 (Plate II.) indicate 

 that the division is very slightly more advanced in the posterior half of 



