stupid question?

Natalie

ok so this maybe a really stupid question but its been playing on my mind and google isnt answering my question as its hard to word haha... so the baby is in the amniotic sac filled with fluid... the umbilical cord is connected to baby and connected to placenta... but the placenta isnt inside the sac.. so how does the umbilical cord get inside the sac without there being a hole?? i hope this makes sense and i hope someone knows the answer haha