975 



but when the interval was fifteen or more seconds, they were pro- 

 duced in greater number. The conclusion deduced from these and 

 similar experiments with, nitrate of potash was, that impregnation is 

 commenced almost at the instant of contact of the spermatozoon 

 with the ovum ; but that duration of contact, and possibly also dif- 

 fluence of the spermatozoon and endosmosis of its substance, is 

 necessary for fruitful impregnation. The experiments were varied 

 by the application of the solution of potash before that of the seminal 

 fluid, in which case the results were more unfavourable. With 

 nitrate of potash, applied before as well as after the seminal fluid, 

 the formation of embryos was not unfrequent. None however were 

 produced when diluted acetic acid was used. This acid acts quickly 

 and most unfavourably on the envelopes of the ovum. 



The agency of the impregnating bodies was then tested in a 

 similar way, by the application of solutions of gum-arabic and of 

 starch, the action of which is merely mechanical. The results were 

 similar to those with the potash. 



When the gum or starch was applied, as in the case of the potash, 

 after the application of seminal fluid in water, embryos were con- 

 , stantly produced, even when the interval between the two applica- 

 tions was only one second ; but when either of these was applied to 

 the ovum before the seminal fluid, then segmentation, if it occurred 

 at all, took place very tardily. In general, however, no segmentation 

 occurred, and no embryos, or but very few indeed, were produced. 



These experiments, compared with those with potash, seemed to 

 show that impregnation is commenced in a very short space of time, 

 and that the spermatozoon is the agent immediately concerned ; and 

 that this agency is material in its operation, as seems to be shown in 

 the fact that it can be prevented by the application both of chemical 

 and of mechanical means to the ovum. We are thus led to infer, 

 that although the spermatozoon does not bodily penetrate into the 

 ovum, its first eff'ect may have some relation to catalytic action, in 

 inducing the segmentation of the yelk ; and, having proof that 

 fluids permeate the coverings of the ovum, we may hereafter find 

 that the process is completed by the diffluence of the impregnating 

 body, and the substance into which it is dissolved, by imbibition 

 into the ovum by endosmosis. 



One plate of the structures described accompanies the paper. 



8. A Mathematical Theory of Magnetism." By Wifliam Thom- 

 son, Esq., M.A., F.R.S.E., Fellow of St. Peter's College, Cambridge, 

 and Professor of Natural Philosophy in the University of Glasgow. 



The Theory of Magnetism was first mathematically treated in a 

 complete form by Poisson. Brief sketches of his theory, with some 

 simplifications, have been given by Green and Murphy in their 

 works on Electricity and Magnetism. In all these writings a hypo- 

 thesis of two magnetic fluids has been adopted, and strictly adhered 

 to throughout. . No physical evidence can be adduced in support of 

 such a hypothesis ; but, on the contrary, recent discoveries, especially 

 in electro-magnetism, render it excessively improbable. Hence it is 



