In Mammalia a thick and highly transparent membrane, — the true chorion, — is formed external to the proper membrane of the yelk, while the latter is in the ovary. The inner part of the substance of the chorion in its early stages is in a fiuid state, so that the 3"elk- ball moves freely in it ; but it subsequently acquu-es more consistence. There is not an}^ structure corresponding to the chorion in the ovary of other vertebrated animals.
The following appears to be the order of formation, as to time, of the more permanent parts of the ovum and the Graafian vesicle in Mammalia, viz. :
1. The germinal vesicle, with its contents, and its envelope of
peculiar granules.
2. The proper membrane of the ovisac, which forms around this
envelope of granules.
3. The yolk, which forms around the germinal vesicle.
4. The proper membrane of the yolk, which makes its appearance
while the yolk is still in an incipient state.
5. The chorion.
The covering or tunic of the ovisac ; and about the same time, the peculiar granules of the ovisac arrange themselves to form,
{The tunica granulosa, The retinacula, and The membrana granulosa.
Such of these structures as are present in the ovary of other Ver- tebrata, appear to originate in the same order as to time.
" Contributions to the Physiology of Vision." By Charles Wheat- stone, Esq., F.R.S., Professor of Experimental Philosophy in King's College, London. Part ilie First. " On some remarkable and hitherto unobserved Phenomena of Binocular Vision."
The author first shovv's that the perspective projections of an ob- ject upon the two retinse differ according to the distance at which the object is placed before the eyes ; if it be placed so distant that to \dew it the optic axes must be parallel, the two projections are precisely similar ; but if it be placed so near that to regard it the op- tic axes must converge, a different perspective projection is present= ed to each eye ; and these perspectives become more dissimilar as the convergence of the optic axes becomes greater. Notwithstand- ing this dissimilarity between the two pictures, which is in some cases very great, the object is still seen single ; contrary to the very prevalent metaph3^sical opinion, that the single appearance of ob- jects seen by both eyes is owing to their pictures falling on corre- sponding points of the two retiuce. After establishing these prin- ciples, the author proceeds to ascertain vdiat would result from pre- senting the two monocular pers23ectives, drawn on plane surfaces, to the two eyes, so that they shall fall on the same parts of the t^vo re- tinae as the projections from the object itself would have fallen. Seve- ral means are described by which this may be accomplished ; but the author especially recommends for this purpose an apparatus called by
