TRENTON — The devastated parents of alleged preppy killer Matthew Hertgen did not appear in a New Jersey courtroom Thursday morning for their son’s first in-person hearing as he faced charges for his younger brother’s beating and slashing death in Princeton, NJ.

Hertgen, 31, appeared coherent and alert during the proceedings in Mercer County Superior Court, and agreed to be held in jail until his trial on murder and animal cruelty charges. He was represented by a public defender — despite his family’s wealth.

Parents David and Debra Hertgen looked utterly bereft at the Saturday funeral for their youngest son, 26-year-old Joseph.

In his eulogy, eldest son David Jr. said, “There’s not one person on Earth that I would be prouder to call my brother.”

Matthew, a bearded former Wesleyan soccer player, is accused of using a knife and a golf club to murder Joseph, who was also a former college soccer player. He then ripped out Joseph’s eye and ate it, according to a law enforcement source.

Sources said he also set the family cat on fire in the rampage at the family’s upscale apartment in Princeton, which is just blocks from the Ivy League university’s campus.

In court on Thursday, Hertgen nodded “yes” as public defender Jason Matey reviewed his constitutional rights and asked him if he consented to his detention.

Mercer County Deputy Public Defender Malaeika Montgomery told The Post she has not heard anything about Hertgen’s family possibly hiring him a private defense attorney — despite his father being the president and CFO of a multimillion-dollar tech company.

When asked what it was like talking to a man accused of such heinous crimes, Montgomery said it was like “talking to any other person.”

“He’s an individual, he’s our client, we’re representing him,” she said. “He’s got a lot of legal options available to him. We’re going to zealously advocate on his behalf.”

“Anyone would be anxious about what’s happening,” she continued. “He’s charged with very serious offenses. So anyone in that situation would be anxious about that. But he’s good, he’s got competent representation, and we’re going to move forward with his case.”

He has seven days to file an appeal if he changes his mind about his pre-trial jail stay, Matey said in court.

The judge set his next court appearance for 1:30 p.m. March 24.

Last Thursday, Hertgen tried to hang himself at the Mercer County Correction Center.

The accused killer faces 30 years to life in prison if convicted.

The former college soccer player embarked upon his twisted rampage — which has stunned those who knew the well-heeled family — months after he posted chilling poems to Facebook that described blood oozing out of eyes and “knives sharpening.”

Hertgen also took a selfie with a cat toy a day before the vicious slaughter. 

The Hertgen brothers were standout athletes who grew up in a $1.1 million home in Toms River before the family uprooted to a ritzy location in Princeton. 

The murder happened last month at the family home in the upscale Michelle Mews apartments, where units can go for up to $2 million.

Officers discovered the grisly scene when the alleged murderer called the cops to report a dead body and fire inside the Witherspoon Street home.

A bloody knife, fork, and plate were found near the victim’s body, a source said, which led cops to believe Hertgen consumed his brother’s missing eye. 

Authorities still haven’t revealed a motive for Hertgen’s alleged spiral into homicidal madness.