GERMINAL MEMBRANE. 41 
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 magnifymg 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 pro- 
duction 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 deci- 
sion of this point is not essential, for the rule that cells 
originate within others is not universal. When the inde- 
pendent 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 represen- 
tation, 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 (Miller’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 
' According to the researches of Martin Barry (Phil. Trans. Part II, 1838, p. 305, 
&c.), both cases appear to occur, so that a cell composed of a structureless mem- 
brane is first formed, (the ovisac of Barry,) and subsequently an external vascular 
covering of cellular tissue. On the relation of this follicle to the mode of develop- 
ment of the oyary itself, see Valentin in Miiller’s Archiv, 1838, p. 526. 
