The author states that this communication to the Royal Society is part of a series of investigations on development, on which he has been for some years engaged, and which was commenced in a paper on that of the Myriapoda, published in 1841, in the Philosophical Transactions. The plan followed in these investigations has been to combine observations on the natural history of the animals with others on the conditions which affect their development, as the best mode of arriving at correct conclusions. The history of the discovery of what can now be proved to be the direct agent of impregnation, the spermatozoon, is then traced; and it is shown, that although within the last few years an opinion has been gaining ground that the spermatozoon, and not the
liquor seminis
, as formerly supposed, is the means of impregnation, no acknowledged proof has hitherto been given of the correctness of this opinion, and no refutation afforded to the theory that the
liquor seminis
is the part of the seminal fluid immediately concerned. The question of the agency of the spermatozoon has thus remained open; and it is to this question, with a view first to supply proof from direct experiments of the fact of the agency of this body, as well as to examine into the circumstances under which this agency is exerted, influenced or impeded, that the present communication is especially devoted. The author then traces the changes in the ovum within the body of the Amphibia, from a short time before the disappearance of the germinal vesicle to the period when the ovum is expelled before impregnation. The structure of the germinal vesicle in the ovarian ovum is shown to be an involution of cells, as stated by Wagner and Barry; but the author differs entirely from the latter respecting the mode of disappearance of the vesicle, and also respecting the part played by its constituents in the production of the embryo. He believes the included cells are liberated by the diffluence of the membrane of the germinal vesicle in the interior of the yelk, not in the centre of the yelk, but much nearer to the upper or dark surface than to the white or inferior, and at the bottom of a short canal, the entrance to which is in the middle of the upper or black surface at a point already noticed by Prevost and Dumas, Rusconi and Boa; and he thinks that it is due to the diffluence of the envelope of the vesicle in this situation that the moment of disappearance has not yet been observed. The germinal vesicle in the Amphibia always disappears before the ovum leaves the ovary, and escapes into the cavity of the abdomen. The mode in which the ovum, after leaving the ovary, is believed to arrive at the entrance of the oviduct is then stated, and the structure of the entrance in the intermedial space, as shown by Swammerdam, described.
On the impregnation of the ovum in the amphibia