BIGELOW: EARLY DEVELOPMENT OF LEPAS. 81 



in which the cleavage furrow will appear ; but later, when the furrow 

 begins to form, the spindle becomes perpendicular to the plane of 

 cleavage. In L. fascicularis the spindle is usually from the very begin- 

 ning of cleavage perpendicular to the chief axis, in which the cleavage 

 furrow later appears. I have noticed the same conditions in the eggs 

 of a species of Balanus. In living eggs of Lepas I have observed move- 

 ments of the egg substances which lend support to the evidence afforded 

 by sections. Figures 8-11 represent conditions between the stages cor- 

 responding to Figures 23 and 24, and they show that the egg under- 

 goes great changes in form before rotation begins. It is probable that 

 the turning of the spindle takes place at the time of contractions of the 

 egg such as those represented in Figures 9-11. 



The astrospheres are well-marked features of the anaphase (Fig. 24), 

 and are distinctly visible as clearer regions in the living egg. 



In a late anaphase the spindle has become straight again and is per- 

 pendicular to the cleavage plane (Fig. 26). The rotation of the ovum 

 is now completed. In this stage the cells are still connected in the 

 centre by a mass of cell-substance, surrounding the spindle (Fig. 26). 



Finally, in the telophase the chromosomes swell into vesicles, and 

 then fuse together to form the nuclei of the two daughter cells in a 

 manner well known for other ova (Figs. 25-27). The cell plate is next 

 completed, and then the separation of the cells (ab\ cd}) is accomplished. 

 Remnants of the spindle may persist for some time, and a well-marked 

 " Zwischenkorper " is often seen. 



Figure 25 represents the condition in the comparatively rare cases 

 in which the cleavage plane remains oblique in an early telophase. 



In observing the living egg it was noted that at the close of the 

 anaphase the protoplasm of the yolk-cell {cd?') is centrally located and 

 that the yolk remains in its original position in the vicinity of the pointed 

 end of the vitelline membrane (Figs. 15, 26). The chief axis of the egg 

 now coincides with the transverse axis of the oval vitelline membrane, 

 the animal pole being marked by the second polar cell, which lies in the 

 cleavage furrow. The formative and nutritive materials of the yolk-cell 

 are not as yet arranged with reference to the chief axis, as they naturally 

 would be if they kept their original relations to the chief axis during 

 the rotation of the dividing ovum. It has been observed that in 

 the living egg the yolk and the central mass of protoplasm move 

 to their respective poles in from twenty to fifty minutes after the com- 

 plete separation of the cells (Figs. 15, 16). It will be seen later that 

 this can have nothing to do with the processes of the second cleavage, 



