JENNINGS: DEVELOPMENT OF ASPLANCHNA HEREICKII. 25 



of the two asters has become centx'al, the other peripheral, as if the cell 

 were about to divide into a deep aud a superficial portion. This condi- 

 tion also is shown in the sagittal section, Figure 7. The difference in 

 the position of the asters in d*-^ and c?*-^ is apparently due to simple 

 mechanical conditions, — the form of the cell d*-^ compelling the asters 

 to take the position which they have. 



At the same time a slight differentiation in the cytoplasm of the cell 

 d*-^ becomes visible. As previously stated, the fine yolk granules are 

 at first distributed uniformly throughout the egg. In this eight-cell 

 stage, a slight concentration of the yolk granules in the ventral part of 

 the cell d'^-^ may be noticed by careful observation. The condition at 

 this time is shown in Figure 7 ; in the ventral part of d*-^ the yolk 

 granules are a little larger and more numerous. As will be shown, this 

 concentration of yolk becomes later much more marked, and its history 

 is peculiar. 



A spindle is now formed in c?*'^ in the position indicated by the asters 

 of that cell in Figure 7, — that is, with a dorso-ventral axis, — thus pre- 

 figuring another equatorial cleavage. The spindle is shown in Plate 2, 

 Figure 16. 



Immediately thereafter the spindle is formed in df*-^, and it appears 

 that the position of the asters shown in Figure 7 (Plate 1) is not defini- 

 tive. The asters shift, so that the spindle in c?*-^ is dorso-ventral, like 

 that in t?*-\ as is shown in Figure 16. Which of the two asters seen in 

 Figure 7 becomes dorsal and which ventral, I have been unable to deter- 

 mine. During the formation of the spindle in d^-'^ the cell extends a 

 little in the direction of the spindle, as is shown by a comparison of 

 Figure 7 with Figure 16. 



Meanwhile, changes have been occurring in the quadrants A^ B, and 

 G. As the processes are the same in all three, the quadrant G will be 

 selected as a type. 



At first, as described above, the asters separate tangentially, at right 

 angles to the axis of the previous spindle (Plate 3, Fig. 17). This 

 position is retained for some time, but in a later stage the line connect- 

 ing the asters in c**"^ has become oblique, as shown in Figure 18, which 

 exhibits a side view of the e^^ of which Figure 7 is a section. The 

 asters in c*-^ still retain their original position. 



Now follows the cleavage of the cell d*-^. This is accompanied by an 

 increase of the dorso-ventral extent of the two products, as compared 

 with that of the original cell. The division is unequal ; the ventral 

 cell d^'^ is much the larger, and retains the whole of the territory con- 



