No. I.] THE EMBRYOLOGY OF CREPIDULA. 55 



spindles which introduce this cleavage have their inner ends 

 at a higher level than the outer ends, and are usually very 

 nearly radial in position, though they are frequently slightly 

 inclined in a right spiral direction, and occasionally even in a 

 left spiral. Whatever may be the direction of the spindles, 

 however, the third cleavage itself is always a dexiotropic one. 

 In the early stages of the formation of these spindles their 

 axes may be radial or even laeotropic ; but usually before 

 the nuclear division is completed, and always before the 

 cell division takes place, the cleavage becomes dexiotropic. 

 Thus in Fig. 12 the spindle in the macromere C is most 

 advanced, while those in D, A, and B show progressively 

 earlier stages in the nuclear division. Now if we consider 

 the outer ends of the spindles as remaining stationary, and 

 the inner and upper ends as being movable, it will be seen 

 that in C the inner end has been rotated slightly in a 

 clockwise direction around the chief axis of the ovum as a 

 centre ; there may be slight indications of this rotation in 

 D and A, though certainly it is not present in B. In other 

 words, the more advanced the cleavage is, the more pronounced 

 the rotation becomes, and what is true in this instance is true 

 in every one that has come under my observation. After 

 the division wall between the dividing cells has appeared, 

 the rotation still continues ; in the formation of both the first 

 and second quartettes there is an actual rotation of these cells, 

 and not merely an oblique cleavage, as is the case in Unio and 

 in some of the cleavages of Nereis. 



In some ova the formation of the micromeres of each 

 quartette takes place in regular succession, as is shown by 

 the successive stages of karyokinesis in the four macromeres 

 of Fig. 12 ; in other ova no such regular succession can be 

 determined. After the micromeres have been separated they 

 continue to rotate until they come to lie in the furrows be- 

 tween the macromeres and alternate with them in position, 

 Figs. 13 and 14. The outer cell walls of the micromeres 

 are at first rounded, as shown in Fig. 13 ; but after they 

 have taken their positions between the macromeres they 

 become pressed down into the furrows so that their outer 



