104 BULLETIN: l^rrSEUM OF COMPAKATIYE ZOOLOGY. 



formed to the begiuning of the transformation of the nucleus and asters 

 into the spindle figure — so that the relative size of the nucleus in the 

 same cell from diflerent eggs gives a measure of its relative age. 



An illustration will make clear the method used. Perhaps the most 

 difficult problem was presented by the movements of the asters and 

 nuclei in tlie two large venti'al cells of the quadrant D, at the fifth and 

 sixth cleavages. These cells are d/''^ and c?°'^ before the fifth cleavage ; 

 d^-^ and d^-^ after the fifth cleavage. Figure 31 (Plate 4) shows the 

 position of the asters in all the cells of this quadrant in the resting 

 sixteen-cell stage. Figure 32 shows the nuclei in quadrant B in the 

 same egg, as seen in a longitudinal section. In the egg shown in Fig- 

 ure 33 the spindles are already completed in quadrant D, indicating 

 that this egg is older than that shown in Figui-es 31 and 32. Figure 

 35 (Plate 5), a ventral view of this stage, shows that the nuclei in a^} 

 and ly''^ are much larger than the nuclei in the corresponding cells of 

 Figui'es 30 and 31, — which likewise indicates that this is an older stage 

 than is represented in the latter figures. Figure 35 shows the exact 

 antero-posterior position of the spindle in d°-'^, while in the other quad- 

 rants the nuclei are shown to be still in a resting condition. 



Figures 37 and 38 together show all the cells of this quadrant during 

 or just after division, the four dorsal cells being still connected by inter- 

 zonal filaments. ' It is therefore a stage later than that shown in Fig- 

 ures 33 and 35. This is in agreement with the larger size of the nuclei 

 in quadrant A, Figure 37, as compared with those of quadrant A in 

 Figure 33, and also with the presence of a spindle in a^-*. In this Qg^ 

 the nucleus in d^-^ (Fig. 38) has resumed its nearly spherical form, and 

 at the side of the cell where the anterior end of the spindle was located 

 (Fig. 35) is a small vesicle, rf®-^ (Fig. 38). This represents one product 

 of the division. 



In d^-^ at this stage (Figs. 37 and 38). the single aster is slightly 

 extended In a direction which is oblique to both the longer and shorter 

 axes of the cell. 



Tlie spindles in the cells of quadrants A, B, and G in Figures 39-42 

 (all from the same egg) prove that this is a later stage than that last 

 considered. In this egg the line joining the two asters in d^-^ (Fig. 42) 

 is oblique to the antero-posterior plane of the embryo, and nearly dorso- 

 ventral. 



In the egg shown in Figures 43-46, division is completed in some of 

 the cells of quadrants A, B, and C, proving this to be a later stage than 

 those shown in Figures 37-42. In this egg the two asters in d^-^ (Fig. 



