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For the first time in U.S. Marine Corps history, women will be training alongside men during boot camp.

A platoon of about 50 female recruits began training Friday at a boot camp with five male platoons in what is traditionally an all-male battalion at the corps' facility at Parris Island, South Carolina, according to a statement.

The integration of women and men at boot camp is "a first in the history of Marine Corps recruit training," the corps said.

The decision was made in the name of training efficiency, according to the corps, and is not a shift in training philosophy going forward. Recruits will be trained by instructors of their gender, according to the corps. About 300 total recruits will participate in the historic session.

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Female Marine recruits fire on the rifle range during boot camp at MCRD Parris Island, South Carolina on February 25, 2013. Scott Olson / Getty Images file

An advocate for women in the military, ret. Army Col. Ellen Haring, who is CEO of Service Women's Action Network, wasn't impressed.

"The Marine Corps' recent announcement that it would integrate an all-female platoon within a company of all-male platoons on a trial basis comes decades after all of the other Services integrated all of their basic training units," she said in a statement.

"After reviewing training data in the 1990s both the Army and the Navy found that integrating basic training units improved performance on a number of measures."

Haring said her organization "has long advocated for fully integrated boot camp in the Marine Corps, including through pending federal litigation alleging that sex segregated training violates the constitutional guarantee of equal protection."

According to the corps, leaders will evaluate the training results after the 13-week session.

"About 50 female recruits, which is smaller than what is typically part of a training cycle, are prepared to begin training at the depot on Jan. 5," the corps' statement reads. "The Program of Instruction (POI) remains unchanged and the recruit training order continues to be the standard for training recruits."

The integrated training "enables appropriate acclimation to the training environment, development of relationships with drill instructors, and focus during the transformation of young women and men into United States Marines," the corps stated.

Haring said separating men and women in training is a tool of "systematic inequality" and argued that the policy correlates to the corps' record of having the fewest women and the "highest rate of sexual assault."

"We encourage the Marine Corps to conduct this experiment transparently, fairly and with independent expert oversight so as to assure sound methodology and reliable results," Haring said.