Page:Microscopicial researchers - Theodor Schwann - English Translation - 1947.pdf/89

GERMINAL MEMBRANE. as a portion of the germinal membrane separating from the whole by a constriction. Both layers contribute to its formation, and it therefore consists of small transparent cells, some of which (probably those pertaining to the mucous layer) contain no nucleus, whilst others (those derived from the serous layer) exhibit the characteristic cell-nucleus with its nucleoli. In addition to these cells it contains a great many nuclei, around which no cells have as yet formed. Between the two layers of the germinal membrane other cells arise, which may be regarded as representing a third layer, the vascular, although they do not really form a connected independent layer; of these we shall treat hereafter. These three layers then, and pre-eminently the first two, form the mediate basis of all the subsequent tissues.

The yelk is not a lifeless aliment for the embryo,—as it is when taken as food by the adult, to whose organism it is dead and must be chemically dissolved,—but the cells of the yelk take part in the vitality called forth by incubation. They effect an alteration in their contents, whereby the albumen which they contain loses its property of coagulating, and the granules become dissolved, in the same manner in which the granules of starch dissolve in the cells of the vegetable embryo. In short, the yelk bears the same relation to the embryo as regards its nutritive property, that the albumen bears to the vegetable embryo.

In accordance with the analogy between the cells we are treating of and those of vegetables, all the changes in the egg, the growth of the germinal membrane, and even the first formation of the embryo, proceed entirely without vessels.