A half-SEM and half-drawing representing the morphological steps of fertilization in Halocynthia roretzi. The egg is enclosed by a chorion (CH) . A single layer of highly vacuolated follicle cells (FC) adheres to the outside of the chorion. Test cells (TC) are located in the perivitelline space.
- The spermatozoon binds to the surface of the chorion by the anterior tip of its head.
- The spermatozoon passes through the chorion with an intact acrosome (A). This suggests that chorion lysin(s) are located on the surface of the plamalemma enclosing the sperm head. The mitochondrion (M) is trapped by the stick-like surface ornamentation which decorates the outer surface of the follicle cells.
- The acrosome reaction (AR) occurs in the perivitelline space.
- In the perivitelline space, apical processes (ApP) protrude only from the apex of the acrosome-reacted sperm head.
- Gamete fusion takes place between some of the apical processes and egg plasma membrane, resulting in the incorporation of the sperm into the egg from the anterior tip of its head.
for details : J. Structural Biology 114, 157-166 (1995)
