GERMINAL MEMBRANE. 41 



perhaps, have had a nucleus. 1 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 arc 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 dcci- 

 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 pcllucida of Baer, chorion of 

 Wagner.) Krause found (Midler'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 



1 According to the researches of Martin Hairy (Phil. Trans. Part II, lS.'iH, 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 tills follicle to the mode of develop- 

 ment of the ovary itself, sec Valentin in Muller's Archiv, 183s, p. 020. 



