JENXIXGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 21 



cell CD', with the undivided aster at its right and slightly behind it 



(Fig- 5). _.^ 



By this time the two asters in the cell AB" have completely separated 

 and lie upon opposite sides of the enlarged nucleus. Thus the prepara- 

 tory stages for karyokinesis are much more advanced in the smaller 

 cell, and it would be anticipated that this cell would cleave first. 



The single aster in CD' now begins to divide. The process seems to 

 be accomplished very quickly, since in a series of nineteen specimens of 

 the two-cell stage (each, of course, taken from a different individual) 

 only one case was found exhibiting a transitional stage between that 

 shown in Figure 5 and that shown in Figure 6. In this specimen the 

 single aster had elongated slightly in the direction of the future 

 spindle. When formed, the spiudle takes an oblique position in the 

 cell, extending from right anterior to left posterior. The aster at the 

 left posterior end of the spindle is much the larger, in correlation appar- 

 ently with the larger mass of cytoplasm surrounding it. The nucleus 

 of CD' has now overtaken in its metamorphosis that of AB" ; the 

 spindles are found in exactly corresponding stages, the chromatin being 

 in both arranged in an equatorial plate (Fig. 6). 



Not only are the two spindles not parallel, as shown in Figure 6, but 

 they do not lie in the same plane. If the two spindles are viewed 

 exactly from the anterior or from the posterior end of the q^^, the left 

 aster in AI^ and the right aster in CD are seen to lie more dorsally 

 than their mates. Viewed in this direction, the spindles cross each 

 other at an angle of about twenty-five degrees. 



As a result of the dissimilarity in the direction of the two spindles, 

 the two next cleavage planes, perpendicular to them, will not meet the 

 first cleavage plane^ in a common line. The position and direction of 

 the spindle in CD' are such that the cleavage plane cutting GLl^ 

 would probably meet the first cleavage plane to the right of the line 

 ■where the plane dividing AB would meet it. Since the right aster of 

 CD' is farther dorsal than the left, the plane of cleavage of CZ)^ 

 would be inclined to the sagittal plane, — on the dorsal side toward the 

 left, on the ventral side toward the right. 



The cleavage of the two cells now fi^llows at almost or precisely the 

 same time, the karyokinetic processes being found from this time on in 

 the same stage. In a series of thirty-one eggs from different individ- 

 aals, each containing more than one and less than five cells, none con- 

 tained exactly three cells. 



An examination of the four-cell stage after the completion of division 



