This picture here is a schematic of what an alveolus looks like at 24 weeks. And the thing that I would like you to focus on is, if you can see it, is the lining of the airway. You can see this nice little layer of epithelial cells, cuboidal epithelial cells. And you think, wow, that looks nice, because you see all these path slides when you are in medical school and you like to see the little cuboidal epithelial cells lined up, but in this situation, if you are trying to breath, it is not a good idea, because these epithelial cells create a barrier that prevents the little capillaries from giving off oxygen and absorbing carbon dioxide. So, as the baby's alveolus goes through the gestational ages, you can see her at 26 weeks, the cuboidal cells are starting to thin out, and then at term, they are all gone. So we have no more block to diffusion. That's what people call a diffusion block. So this is sort of what you might see if you had a premature baby, and you were able to look at a cross section of lung tissue, you would see that diffusion block in the earlier gestational ages that goes away as you get closer to term.