Simplify the following text for a Grade 6 native English-speaking person.
Original: The mouth of newly born metatherians is fused laterally , but open medially ; this forms an `` O '' shaped mouth in which the mothers nipple fits , then swells to secure the offspring into place for further development and growth .
Simplified: The mouth of newly born metatherians forms an `` O '' shape into which the mother 's nipple fits . Then it swells to secure the offspring into place .