230 Royal Society .— 



the principal results of experiments made during the months of 

 June and July last, on the impregnation of the ovum in Gasterosteus 

 leiurus and G. pungitius, and hope to be able to furnish a more 

 detailed account of my observations on a future occasion. 



The ovarian ovum of these fishes, at a very early stage of its 

 development, is provided with a proper investing membrane, the 

 future chorion. At a later period, one portion of this membrane 

 presents a number of cup-shaped pediculatcd bodies scattered over 

 its surface, and in the centre of this part of the chorion there is a 

 funnel-shaped depression, pierced by a canal which leads towards the 

 centre of the egg. 



In the nearly ripe ovum, the germinal vesicle occupies an excentric 

 position Avith respect to the egg as a whole, but imbedded in the 

 centre of a semi-solid accumulation of fine granular matter at that 

 part of the surface which corresponds to the funnel-shaj)ed depres- 

 sion ; so that the apex of the funnel, projecting inwards beyond the 

 level of the inner surface of the chorion, makes a depression in the 

 centre of the layer of granular matter, and comes nearly into contact 

 with the germinal vesicle. 



For convenience of description, the funnel shaped depression will 

 now be called micropyle, and the layer of granular matter before 

 impregnation, discus proligerus. 



The germinal vesicle disappears before the ovum leaves the ovary, 

 and no remnant of it or its spots can be seen. 



A very delicate membrane invest? the yelk within the chorion ; 

 this membrane is more distinct after impregnation, or after the 

 action of water upon an unimpregnated egg ; it may be isolated, and 

 then exhibits a remarkable degree of elasticity. It is not a yelk-mem- 

 brane, and it will be spoken of as the inner memhrane. 



The layer of the yelk immediately internal to the inner membrane 

 passing over the discus proligerus, is formed by yellowish highly 

 refractive drops which disappear in water, undergoing some remark- 

 able changes, and by a fluid substance which water precipitates in 

 a finely granular form. 



The principal mass of the yelk consists of a clear and very con- 

 sistent albumen. Ihe oil is collected into a few very large drops 

 which come up to the surface. 



When the ovum escapes from the ovary, it enters a cavity which 

 may be considered as the ovarian extremity of the oviduct, in which 

 a considerable quantity of clear viscid fluid is previously secreted 

 and collected, to be expelled with the ova. 



More exact observation of the micropyle in the free eggs proves 

 that the inner end of the canal is either open, or at most closed by 

 a very delicate membrane. When looking into the funnel from the 

 wide mouth, the apex being in focus, a bright, clear, round or oval 

 spot, such as an aperture would produce, is always visible. If a 

 section be made of the egg, and the apex brought into focus from 

 within, the same clear spot is well seen, and the fine and regularly 

 dotted structure of the chorion is seen to cease suddenly at the 

 margin of the clear spot. 



