Mitochondria, rough endoplasmic reticulum and ribosome are all inthe top ofthe cells and increased in number with the development of thelungs$Canalicular stage(13-l4wks):The respiratory bronchioles developedquickly the terminal buds?

• 小管期（第13-14周）以呼吸部发育为主，终蕾腺状结构逐渐消失，终蕾上皮细胞由高矮不等的单层柱状上皮逐渐演变为立方形的原始肺泡上皮，原始肺泡开始形成；