KOFOID : DEVELOPMENT OF UMAX. 55 



cells of the basal quartet (5.1) retain their preponderance, and may still 

 he designated as macromeres after this division. 



The cell cZ 5 ' 2 (somatoblast of Wilson) is not appreciably larger than 

 the other members of the quartet to which it belongs. 



Division of Quartet 4.2, forming 5.3 and 5.4. 



Plate III. Figs. 20-33 ; Plate IV. Figs. 28, 29. 



The same stages which show spindles in the lower quartet also exhibit 

 them in the upper and smaller quartet. The mitosis is not however so 

 far advanced as in the lower quartet. The nuclear membrane can still 

 (Figs. 20, 21) be traced, though the asters are present, and the axis of 

 the spindle can therefore be determined. This, as in the lower quartet, 

 is inclined ; however, it is more nearly parallel to the equator than to 

 the vertical axis. The inclination is in the same general direction as that 

 of the spindles of the lower quartet, and the conditions of the completed 

 division can be inferred as readily from the figures. Viewing the egg 

 from the animal pole and passing from the lower derivative or aster to 

 the upper, we move in a direction opposite to that of the hands of a 

 clock, and this spiral, like that of the other quai'tet of this generation, 

 is therefore a left one. This division, like that of the basal quartet, 

 results in cells of almost equal size (Plate IV. Figs. 28, 29), the upper 

 derivatives in this case (5.4) being, however, slightly larger than the 

 lower (5.3). 



The conditions in Figures 20, 21, show that the sixteen-cell stage will 

 in this case follow immediately upon the eight-cell, without the inter- 

 vention of a well marked twelve-cell stage. There is, however, so much 

 variation in the rate of cleavage in Limax, that it might be expected that 

 a twelve-cell stage would occasionally make its appearance. We have 

 but to increase the difference between the mitotic conditions of the cells 

 of the two quartets of Figures 20 and 21 to produce such a stage. 

 Warneck ('50) figures in Tafel V. Fig. 46, a twelve-cell stage of Limax 

 agrestis, and one egg in this stage has come under my own observation. 

 This stage occurs regularly in forms with abundant yolk, as Planorbis, 

 Umbrella, etc., but Nereis, like Limax, passes directly from the eight- to 

 the sixteen-cell stage. The completed sixteen-cell stage is shown in 

 Figure 22, Plate III., in which the genetic relations of the cells are still 

 indicated by the approximated nuclei. 



With the completion of the sixteen-cell stage and the fifth generation, 

 the dorsal and ventral cross furrows are restored to the conditions of 



