THE BIOLOGY OF THE CELL SURFACE 



trulation) when it occurs — In eggs with buds equal to or 

 smaller than the egg-component having ectoplasm — takes 

 place at the pole where the bud is formed. This fact might 

 mean that the endoplasm is always extruded from the area 

 in the uninseminated egg which is destined to be the site of 

 invagination. But it is also probable that the endoplasm 

 extrudes from any region of the egg, which means that the 

 site of invagination becomes pre-determined by extrusion 

 of the endoplasm. Whilst sea-urchins' eggs lend them- 

 selves beautifully to this mode of endoplasmic extrusion, 

 other eggs do not because of the nature of their surfaces 

 and the changes taking place in them after experimental 

 treatment. In these the vitelline membranes must be 

 punctured before the endoplasm can protrude. Such 

 injury leads at once to break-down of the egg-cytoplasm, 

 as happens when one cuts into the tough membrane 

 enclosing the egg of Nereis. 



In the Q^^ of the starfish one can demonstrate that the 

 egg-surface plays in fertilization a role similar to that of 

 the surface of sea-urchins' eggs. Observations reported 

 by Whitaker^ make it clear that in fertilization of the star- 

 fish Qg%, the effect of the spermatozoon is conducted only 

 by the egg-surface. 



The conclusion reached from these observations, that the 

 ectoplasm is necessary for fertilization, though it is defi- 

 nitely proved for only a few eggs, may nevertheless hold 

 for eggs generally, since in them all some kind of surface- 

 change follows sperm-attachment. We should not, to be 

 sure, make a virtue of the necessity imposed upon the 

 spermatozoon that to effect fertilization it must first make 

 contact with the egg-surface; we must know that this par- 

 ticular surface is something peculiar because of special 

 endowments which set it apart from the endoplasm. 



^ Whitaker^ igji. 



194 



