Page:Microscopicial researchers - Theodor Schwann - English Translation - 1947.pdf/67

Rh perhaps, have had a nucleus. Within this vesicle lies the ovum or vesicle of Baer, embedded in a layer of granules. When these granules are examined with a magnifying power of 450, they are readily recognized to be cells, that is, round vesicles containing a nucleus, which is situated upon the internal surface of the wall. The nucleus being granulous and darker than the rest of the object falls under observation first. It encloses one or two nucleoli. The cell surrounding it varies in size, being in the average about half as large again in diameter, but some are much larger. The cells are for the most part extremely delicate, and round, when separated from one another. When in connexion, they often flatten against one another, and assume a polyhedral form. In addition to these cells, isolated nuclei appear also to be present within the Graafian vesicle, perhaps as the germs of new cells. The production of these cells proceeds according to the fundamental law mentioned at page 39, within the fluid of the Graafian vesicle, that being their germinative material or cytoblastema. Whether this fluid is to be regarded as cell-contents, and the cells produced in it as being formed within a parent cell, must depend upon the solution of the question, as to whether the Graafian vesicle be an elementary cell or not; but the decision of this point is not essential, for the rule that cells originate within others is not universal. When the independent vitality of cells is borne in mind, we can readily conceive how these, when they (after the bursting of the vesicle) arrive with the ovum in the uterus, may be further developed into other structures (the chorion according to Krause.) Within this granulous or rather cellular disc then the ovum or vesicle of Baer lies embedded, (see the representation, plate II, fig. 1, taken from Krause.) The first object which attracts observation is the dark spherical yelk, surrounded by a transparent space, (zona pellucida of Baer, chorion of Wagner.) Krause found (Müller’s Archiv, 1837, p. 27) that the yelk is surrounded by a peculiar membrane, d (vitelline membrane),and that the transparent space is enclosed externally