Blastoderm

Blastoderm, an embryological term applied to the layer or layers of cells arising from the germinal disc, or the portion of a partially segmenting egg which undergoes division. In ova where there is a large quantity of nutritive material or yolk, as in fish or bird, the whole ovum cannot divide, and only a small (germinal) disc of 'formative protoplasm' does so. The cells resulting from the division of this area become afterwards disposed in the ordinary germinal layers, and are in their earlier stages, as they grow round the yolk, and become in their area of origin the seat of embryonic development, called the blastoderm. See EMBRYOLOGY.