with the ovisac, had constituted the Graafian vesicle. It is the vascular covering of the ovisac w^hich becomes the corpus luteum. Many ova, both mature and immature, disappear at this time by ab- sorption. In some animals minute ovisacs are found in the infundi- bulum, the discharge of which from the ovary appears referable to the rupture of large Graafian vesicles, in the parietes or neighbour- hood of which those ovisacs had been situated.
The diameter of the rabbit's ovum, when it leaves the ovary, does not generally exceed the 135th part of an inch, and in some in- stances it is still smaller. The ovum enters the uterus in a state very dilierent from that in which it leaves the ovary ; hence the opinion, that "in their passage through the tube the ova of Mam- malia undergo scarcely any metamorphosis at all," is erroneous. Among the changes taking place in the ovum during its passage through the Fallopian tube are the follov^ing ; viz. 1. An outer membrane, the chorion, becomes visible. 2. The membrane origin- ally investing the yelk, which had suddenly thickened, disappears by liquefaction ; so that the yelk is now immediately surrounded by the thick transparent membrane of the ovarian ovum. 3. In the centre of the yelk, that is, in the situation to which the germinal vesicle returned before the ovum left the ovary, there arise several very large and exceedingly transparent vesicles : these disappear, and are succeeded by a smaller and more numerous set ; several sets thus successively come into view, the vesicles of each succeeding set being smaller than the last, until a mulberry-like structure has been produced, which occupies the centre of the ovum. Each of the ve- sicles of which the surface of the mulberry-like structure is com- posed contains a pellucid nucleus ; and each nucleus presents a nu- cleolus.
In the uterus a layer of vesicles of the same kind as those of the last and smallest set here mentioned makes its appearance on the whole of the inner surface of the membrane which now invests the yelk. The mulberry-like structure then passes from the centre of the yelk to a certain part of that layer, (the vesicles of the latter coalescing with those of the former where the two sets are in contact to form a membrane,) and the interior of the mulberry-like structure is now seen to be occupied by a large vesicle containing a fluid and gra- nules. In the centre of this vesicle is a spherical body ha\ing a granulous appearance, and containing a cavity apparently filled with a colourless and pellucid fluid. This hollow spherical body seems to be the true germ. The vesicle containing it disappears, and in its place is seen an elliptical depression filled with a pellucid fluid. In the centre of this depression is the germ, still presenting the ap- pearance of a hollow sphere. The germ separates into a central and a peripheral portion, the central portion occupies the situation of the future brain, and soon presents a pointed process which is the rudi- ment of the spinal cord. These parts at first appearing granulous are subsequently found to consist of vesicles.
Thus the central portion of the nervous system is not originally a fluid contained within a tube, but developes itself in a solid
