In a memo sent to employees, David Levin, the president and chief executive of McGraw-Hill Education, issued an apology, calling the caption “a mistake” and saying the company was reviewing its internal procedures and increasing its list of textbook reviewers to reflect greater diversity.

“We are deeply sorry that the caption was written this way,” Mr. Levin wrote to employees. “While the book was reviewed by many people inside and outside the company, and was made available for public review, no one raised concerns about the caption. Yet, clearly, something went wrong, and we must and will do better.”

In an interview, Mr. Levin said the textbook, which spans more than 800 pages, does not sugarcoat the issue of slavery and includes more than a dozen references to the capture and enslavement of Africans. He said the caption was written in 2012 and had been posted, along with the rest of the book’s content, on a Texas website as part of the state textbook adoption process for almost a year. No objections to the caption were raised, he said.

There were more than 100,000 copies of the textbook in the hands of Texas school districts. Mr. Levin said the company was in touch with districts and was offering to replace the textbook, provide a sticker with the rewritten caption to cover up the old one or supply a lesson plan free of charge to teachers on cultural sensitivity “to create an opportunity for a richer dialogue.”

A spokeswoman for the Pearland Independent School District said high school social-studies teachers would not use the textbook when teaching that part of the class until the publisher made the updates. The teachers will use “different resources in teaching that content” in the meantime, said the spokeswoman, Kim Hocott.

At Pearland High School, nearly 800 students use the World Geography textbook, the latest edition of which was put into use this school year. In addition to the online versions of the textbook the students used, there were 244 print-edition sets at the school, said Ms. Hocott, who suggested that it was the teachers and not the textbook that guided the instruction in the classroom.

“In Pearland I.S.D., textbooks are used as a resource and do not drive the curriculum,” she said in a statement. “We’re proud that our teachers serve as the primary resource for information on the curriculum in which they teach.”