10 OUTLINES OF CHORDATE DEVELOPMENT 



normal nucleus together with sperm centrosome and aster, the 

 second polar body is formed and the mature egg nucleus 

 established. The second polar body forms about fifteen 

 minutes after entrance of the sperm. The division of the 

 sperm centrosome and aster, the approach and fusion of the 

 egg and sperm nuclei, and the constitution of the first cleavage 

 nucleus, are in no way unusual (Fig. 4). 



The first cleavage spindle takes up a position definitely 

 related to the symmetry of the egg, lying horizontally, slightly 



FIG. 4. Prophase of first cleavage figure in Amphioxus. After Sobotta. 

 Inner and outer membranes fused and separated from the egg by a wide space 

 called the perivitelline space. 



nearer the animal pole, and at right angles to the median plane. 

 The relation of this symmetry of the egg to that of the future 

 embryo is shown in Figs. 2 and 7, A. The median plane of 

 symmetry is determined by the points of polar body formation 

 and sperm entrance, and the eccentric position of the clear 

 protoplasmic region. This last is displaced toward the antero- 

 dorsal region of the future embryo: one result of this is the 



