STUDY OF CELL MECHANICS 451 



The protoplasmic streaming is extremely variable not only 

 in the eggs of different females, but in the eggs of one individual. 

 The typical course of events is shown in the camera and free- 

 hand sketches reproduced in figures 1 to 5 and 11, 12, 14. But 

 in some eggs the aster does not flatten so much as to become 

 concave, nor does it move very far in the egg substance. Going 

 hand in hand with this, the movement of the protoplasm is not 

 so severe. A full discussion of such eggs will be given later. 



One very striking phenomena" which accompanies the pro- 

 toplasmic movement, is the flow of the material from the cor- 

 tical layers of the egg into the protoplasmic processes. This 

 is best seen in heavily pigmented eggs, such as those of Arbacia, 

 from which figure 11 was made. The same flow is seen in 

 Strongylocentrotus, although it is not conspicuous because of 

 the lack of pigment. 



The time when the movement of the protoplasm is seen is 

 extremely variable when measured in terms of the control eggs. 

 It may be observed anywhere between the 2- and 8-cell stage. 

 But in any single egg it only follows after certain definite 

 changes in the aster and in the chromosomes. The length of 

 time which the movement lasts is also variable. Sometimes 

 the egg will be rounded up twenty minutes after the swelling 

 of the membrane began, or it may be double that time before 

 the period of movement is over. 



The second division sets in, usually, just as the normal 

 rounded form is being resumed (fig. 14) and the divergence 

 which various eggs show in their behavior (see p. 449) begins. 



In the large majority of cases, the second division cycle is 

 accompanied by the formation of an amphiaster (figs. 12 and 

 14). Except for the excentric position of the spindle and the 

 large asters, such eggs appear normal. When cytoplasmic 

 cleavage sets in, however, we do not have the clean cutting 

 through of the protoplasm. Instead, there is first a very pro- 

 nounced swelling of the ectoplasmic layer in the cleavage plane, 

 which amounts to a disintegration, often, on the side farthest 

 away from the spindle (fig. A). This is followed by severe 

 protoplasmic movements in the cleavage plane (figs. Ai, A 2 



