I'm so excited to have found an answer for my son's question.  We do our studies with Weaver and we are on Volume 1 Chapter 12 Human Body, Life Cycle, and Family.

DS11 was wondering if boys had one less rib than girls.  It's frustrating not being able to answer his question, but in return I told him that I would be interested in finding out that answer also.  As usual, God is so faithful.  I had to run to the store today and happened to listen to a short program with Ken Ham.  They had said about a free pamphlet on creation... which I thought might work good with our unit.  So, I came online to check it out.  Low and behold I seen the free article on Adam's rib.  Check it out here. Absolutely amazing... I never heard of periosteum before.  I never heard that our ribs could grow back.  How interesting is that???  I liked his original answer also... I don't know why I didn't think to say to ds11 that he was created having one less rib... but God had taken that rib from him to create Eve.  Thus, when his children would be born they would have all of their ribs. 

I have soo much to learn.  Thank you Father God for continuing to teach me in ways that I can learn.