CYTOKINESIS. 79 



That the movements within the cell substance of the unsegmented egg are, in 

 certain cases at least, of a vortical character is indicated by spiral asters, first de- 

 scribed by Mark for Limax, and since observed by several investigators in other 

 animals, and also by my observation that the first cleavage in Crepidula is a spiral 

 one, being oblique to the right, or dexiotropic, (see p. 80, also Conklin '97). 



C. Movements During Cleavage. 



It is, however, in the cleavage of the egg that I have found the most unmis- 

 takable evidences of definite and orderly movements of the cell contents. These 

 movements occur before, during and after the division of the nucleus, and are thus 

 characteristic of the entire cycle of division. Since the different cleavages differ 

 considerably in the character and extent of these movements it will be necessary to 

 devote some attention to each cleavage. 



The entire history of these movements could never be determined by means of 

 sections alone, though these are of great supplementary value, but recourse must be 

 had to preparations of entire eggs. In such eggs, prepared in the manner specified 

 on page 6, the whole course of these movements can be followed with great clear- 

 ness and the relative positions of spheres, ceritrosomes, nuclei and mid-bodies can 

 be accurately determined at every stage. 



(1). First Cleavage. — At the beginning of this cleavage the cytoplasm is well 

 separated from the yolk in the region of the germ nuclei. Above the nuclei and 

 below the polar bodies traces of the egg and sperm spheres may still be seen, figs. 78, 

 79. As the spindle elongates and the astral radiations extend, the cytoplasmic area 

 first elongates and then the entire egg becomes ellipsoidal. 



From the prophase to the anaphase the mitotic figure lies in a cytoplasmic area 

 almost entirely free from yolk and there are few, if any, yolk spheres between the 

 spindle and the polar bodies, figs. 55, 57, 58 (in the preparation from which figs. 57 

 and 58 were made, the polar bodies were attached as in figs. 59-61, but in order to 

 save space on the plate the upper parts of the figures, showing the polar bodies, 

 were cut off from the drawings). In the late anaphase, however, the yolk spheres 

 are present not only in the superficial layer of protoplasm, but also in a plane run- 

 ning right through the middle of the spindle, figs. 59, 60, 66, 67; in fact the only 

 area free from yolk spheres at this stage is that immediately surrounding each of 

 the asters. This position of the yolk can have been caused only by extensive move- 

 ments of the cell substance, the yolk being carried up at the periphery toward the 

 animal pole and then down through the middle of the egg in a plane at right angles 

 to the axis of the spindle {cf. figs. 55-61, 80, 81). While this movement is most 

 easily seen and is probably strongest in the direction indicated, i. e., up at the 

 periphery toward the animal pole and then down through the plane of the first 

 cleavage, yet the constriction which forms all around the egg (see fig. 80) shows that 

 the movement must be from the whole equatorial periphery toward the spindle axis. 

 However, this constriction is deepest on the side of the animal pole where this move- 

 ment is most evident. 



