The extraordinary resistance to disruption shown by the eutherian sperm nucleus is associated with a significant complement of ‐S‐S‐ crosslinks within the chromatin. The present light and electron microscopic study, in which the spermatozoa of many vertebrates and some invertebrates were exposed to SDS with or without dithiothreitol, demonstrates that the extreme degree of disulfide bonding in the sperm chromatin of eutherian mammals is highly unusual in a comparative sense. In no other species did the sperm nucleus display a resistance to disruption comparable to that in eutherians; on the contrary, in most groups including prototherian and metatherian mammals, such crosslinks are absent or relatively sparse, the greatest degree of crosslinking outside the eutheria being seen in octopus and skate.

The suggestion is advanced that the unusually thiol‐rich protamines have evolved as a structural adaptation which, through crosslinking, endows the sperm head with a rigidity of possible importance in the penetration phase of fertilization. This seems likely because (1) any degree of ‐S‐S‐ determined stability present or inducible in sperm chromatin often appears more intense in or is confined to the anterior region of the nucleus (e.g., newt, snail, turtle and immature eutherian spermatozoa); (2) the zona pellucida, which the fertilizing sperm must penetrate, is an unusually thick, compact and resilient coat in the eutherian ovum in particular; and (3) the occasional observation of a sperm nucleus buckled within the pathway of its penetration into the rabbit zona pellucida suggests that in eutheria this tough vestment presents a relatively rigid barrier to the sperm head.

Its apical location suggests a similar structural role for the ‐S‐S‐ stabilised perinuclear projection, or “perforatorium” around the leading border of the sperm nucleus in eutherians as well as in some non‐eutherian species, such as octopus, snake and turtle.