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The outer layer of the blastoderm; the epiblast; the ectoderm.

o a germ; as, the germinal vesicle. Germinal layers (Biol.), the two layers of cells, the ectoblast and entoblast, which form respectively the outer covering and inner wall of the gastrula. A third layer of cells, the mesoblast, which is formed later and lies between these two, is sometimes included. -- Germinal membr…

, the segmentation cavity in which the mesoblast is formed lies between the entoblast and ectoblast. See Illust. of Invagination. -- Segmentation nucleus (Biol.), the body formed by fusion of the male and female pronucleus in an impregnated ovum. See the Note under Pronucleus. -- Segmentation of the ovum, or Egg clea…