lecithoblast

例句：

1. 1. The development of the fetus is closely related to the growth of the yolk sac.
   胎儿的发展与卵黄囊的生长密切相关。
2. 2.During embryonic development, the yolk sac provides nutrients for the developing fetus.
   在胚胎发育过程中，卵黄囊为发育中的胎儿提供营养。
3. 3. Researchers have found that the yolk sac plays a crucial role in the immune system development of the fetus.
   研究人员发现，卵黄囊在胎儿免疫系统发育中起着至关重要的作用。