yolk endoderm

例句：

1. 1.The development of the embryo is divided into three stages: ectoderm, mesoderm, and endoderm.
   胚胎的发展分为三个阶段：外胚层、中胚层和内胚层。
2. 2. During the early stages of development, the endoderm forms the digestive system.
   在发育的早期阶段，内胚层形成了消化系统。
3. 3. The endoderm plays a crucial role in the formation of the nervous system.
   内胚层在形成神经系统中起着至关重要的作用。