Suffolk’s chief medical examiner gave a biology lesson Monday about the durability of sperm cells to a jury in a double homicide trial.

The point of the lesson — complete with photos of sperm cells collected from the bodies of the two victims — was to establish how soon after sex the two women were killed. Dr. Michael Caplan testified during the trial of John Bittrolff, 50, a Manorville carpenter charged with two counts of second-degree murder.

He is accused of strangling and bludgeoning Rita Tangredi, 31, of East Patchogue on Nov. 2, 1993, and Colleen McNamee, 20, of Holbrook on Jan. 30, 1994. Both women worked as prostitutes, and Bittrolff’s DNA was recovered from semen found in both bodies.

During questioning by Assistant District Attorney Robert Biancavilla, Caplan said he used a microscope to examine cells swabbed from various body parts of both victims. The density of sperm cells he found in different areas told him how long before death they could have been left there, he said.

Caplan said the sperm found in Tangredi’s body could have been there no longer than 26 hours.

For McNamee, Caplan said the sperm on one sample could have been there no more than 24 hours, he said.

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In both cases, he said the sperm could have been there a much shorter period of time, but there’s no way to determine that.

The defense is expected to argue that the 24-hour window between sex and death leaves plenty of time for someone other than Bittrolff to be the killer. Defense attorney William Keahon of Hauppauge also argued in his opening statement that “sexual relations don’t equal a killer.”

Earlier Monday, Caplan testified about McNamee’s autopsy. He recounted Tangredi’s autopsy last week before state Supreme Court Justice Richard Ambro.

As with Tangredi, Caplan said McNamee suffered enormous head injuries. He compared the many broken bones in her skull and face to a crushed egg.

In both cases, the blows were so severe that brain matter was forced from their skulls. Caplan said material found a few feet from Tangredi’s body was brain matter. The autopsy of McNamee found that “at least a third of her brain matter was missing,” he said.

Both victims had some cocaine in their bloodstreams, he said. Tangredi had alcohol in her stomach but hardly in her blood, indicating she was killed almost as soon as she drank it, he said.