Vitellus, the Yolk, first named and fully illustrated by Gærtner, is less general than any of the parts already mentioned. He characterizes it as very firmly and inseparably connected with the Embryo, yet never rising out of the integuments of the seed in germination, but absorbed, like the Albumen, for the nourishment of the Embryo. If the Albumen be present, the Vitellus is always situated between it and the Embryo, and yet is constantly distinct from the former. The Vitellus is esteemed by Gærtner to compose the bulk of the seed in Fuci, Mosses and Ferns, as well as in the genus Zamia, closely allied to the latter, see his t. 3, and even in Ruppia, Engl. Bot. t. 136, and Cyamus. In the natural order of Grasses the part under consideration forms a scale between the Embryo and the Albumen.
I cannot but think that the true use of the Vitellus may be to perform the functions of a Cotyledon with regard to air if not to light, till a real leaf can be sent forth, and that the "subterraneous Cotyledons" of Gærtner in the Horse Chesnut