KOFOID: DEVELOPMENT OF LIMAX. o1 
meeting in a point at the animal pole, while the ventral eross furrow is 
correspondingly longer. In Umbrella (Heymons '93) the dorsal and 
ventral furrows of this stage are parallel, i. e. are formed between the same 
cells B and D, the cells A and C being considerably separated. This is 
undoubtedly due to the presence of a large amount of yolk in the four 
blastomeres. Likewise in Planorbis (Rabl '79) and Neritina (Blochmann 
'81) we find the dorsal and. ventral furrows of this stage similar to those 
of Umbrella, rather than Limax. These cross furrows are an invaluable 
aid in the determination of the axes of the later stages; tho question of 
their relations and constancy will be discussed later. 
FOURTH GENERATION. ThirD CnEkAvAGE. Eran CELLS. 
Plate I. Figs. 8-13 5 Plate II. Figs. 17-19; Plate III. Figs, 20, 21. 
About two hours and a half intervene between the beginning of the 
four cell stage and that of the cight-cell stage. The third cleavage is 
accomplished by the division of the quartet of the third generation, A, B, 
C, D, into two superposed quartets (cf. Figure D, p. 52), AU — D, 
and a? — d*?, The series of stages shown in Figures 8-13 (Plate I.) 
represents tho egg during this process, Figures 17 and 18 (Plate II.) 
givo apical and lateral views respectively of an egg with the spindles of 
this generation. It will be noticed (Fig. 17) that the spindles in no 
case stand vertically, but that they are inclined toward the rigt (right 
aud left being used as resident in the egg; see my earlier paper, 794, 
p.180). The division of the chromatin elements in the spindles has 
just begun, and there is no trace of a constriction of the cytoplasm. A 
comparison of Figures 17 and 19 shows that the degree of obliquity of 
the spindles has increased during the interval between the two stages. 
Figure 19 represents a stage in which the constriction of the upper from 
the lower quartet, i. e, of the micromeres from the macromeres, has just 
been completed. The obliquity increases during the division, so that at 
its close the cells of the upper quartet lio in the furrows between the 
cells of the lower quartet. Thus it will be seen that this apparent 
shifting of the upper quartet upon the lower, known as the “spiral,” 
takes place in large measure during the division of the cells. It will 
also be noted that the plan of division is at right angles to the axis 
of the spindle. 
If we view the egg from the animal pole and pass from the lower 
derivative of a pair to the upper, we move in the direction of the hands 
of a clock, and thc crore the division of this, the fourth generation, takes 
