No. 2.] EARLY DEVELOPMENT OF PLANORBIS. 379 



There is doubtless an error in the orientation of this figure, if not 

 in the figures of the cross furrow in some of the preceding stages. 

 In P. trivolvis the upper polar furrow would lie at right 

 angles to the lower one, were it not that the shifting of the 

 micromeres from right to left lessens this angle to one of about 

 45°. The eight-cell stage in P. trivolvis differs from that fig- 

 ured in Kofoid's Fig. D in that the upper polar furrow lies 

 between \a and i^(or the cells which Kofoid has called b*'"^ and 

 d^''') instead of i^ and id {a*'"", b*'"", Kofoid), and is placed at right 

 angles to the one there figured. The upper polar furrow makes 

 a positive instead of a negative angle of 45° with the lower 

 polar furrow taken as an axis. 



From the Eight to the Twenty-four Cell Stage. 



The four macromeres are the next cells to divide. This 

 cleavage is dexiotropic, i.e., in the reverse direction to the 

 previous one. The second quartette of ectomeres thus formed 

 is, like the first, composed of rather large clear cells, which, 

 however, are markedly smaller than the macromeres. The cells 

 flatten out after division, as after the preceding cleavages, so 

 that the furrows between the cells almost disappear, and the 

 cell outlines are marked only by narrow clear lines. 



From the twelve-cell stage the ^gg passes quickly to that of 

 twenty-four cells. The first quartette of ectomeres are the first 

 cells to divide, the division occurring in a right-handed spiral. 

 Very soon spindles occur almost simultaneously in the cells of 

 the second quartette and in the macromeres. The cleavage of 

 the cells of the second quartette is laeotropic, and the resulting 

 cells are nearly equal in size. The division of the macromeres, 

 which is likewise laeotropic, gives rise to the third generation 

 of ectomeres. The cells of the third generation are large, and 

 are marked off sharply from the macromeres, even before the 

 division is completed, by their clear protoplasm. From their 

 superficial outline they appear equal in size to the macromeres, 

 or even larger. Their actual bulk, however, is less, as may be 

 seen in optical sections, for they do not extend so far into the 

 interior of the Q.g%. 



