Apparently the book is online:
There was an uneasy silence. Several of the boys blushed. They had not yet learned to draw the significant but often very fine distinction between smut and pure science. One, at last, had the courage to raise a hand.
"Human beings used to be …" he hesitated; the blood rushed to his cheeks. "Well, they used to be viviparous."
"Quite right." The Director nodded approvingly.
"And when the babies were decanted …"
"'Born,'" came the correction.
"Well, then they were the parents–I mean, not the babies, of course; the other ones." The poor boy was overwhelmed with confusion.
"In brief," the Director summed up, "the parents were the father and the mother." The smut that was really science fell with a crash into the boys' eye-avoiding silence. "Mother," he repeated loudly rubbing in the science; and, leaning back in his chair, "These," he said gravely, "are unpleasant facts; I know it. But then most historical facts are unpleasant."