MICRODISSECTIOX STUDIES. 335 



through such an egg quickly closes up. The cortex i.e., the sur- 

 face of the egg immediately beneath the egg membrane tends 

 always to remain more solid (Chambers, '17-0). Because of this 

 difference in consistency the cortex and medulla of the egg can be 

 separated from one another as follows ('2i a ) : If the surface of 

 the mature starfish egg be torn with a needle and the egg then 

 be caught at the opposite side and pulled to the edge of the 



FIG. 1 8. Currents produced within a sea-urchin egg by pushing a sea- 

 urchin egg held against a coverslip by a shallow film of water. The direction 

 of the currents is shown by the arrows. The nucleus, after being carried 

 about with the current, tends to come to rest in the location shown in the 

 figure. 



FIG. 19. Part of the cortex of a fertilized egg after the appearance of the 

 hyaline plasma layer. The cortex was ruptured in one place and cytoplasmic 

 granules can be seen issuing through the rupture in the hyaline plasma layer 

 and the investing fertilization membrane. 



hanging drop, the compression on the egg produced by the shal- 

 low water at the edge of the drop will cause the fluid interior 

 to ooze out through the tear to form a spherical exovate (see 

 Fig. 25, page 344). One may so manipulate the process as to 

 cause the egg nucleus either to remain behind in the cortex (the 

 cortical remnant) or to pass into the extruded sphere of endo- 

 plasmic material. 



The cortical remnant is relatively solid and remains more or less 

 inclosed within the egg membrane and its jelly. If left long 

 enough it will eventually round up so as to present the appearance 

 of a diminutive egg surrounded by a collapsed and wrinkled egg 

 membrane. 



