973 



stances which affect this, are then shown to depend on the degree 

 of expansion of the envelopes, the imbibition of fluid, the tempera- 

 ture of the surrounding medium, and the degree of aeration. The 

 envelopes expand and imbibe fluid most rapidly during the first half 

 hour after the egg has been laid, and the susceptibility is diminished 

 in the inverse ratio of the expansion and imbibition. It is greatest 

 during the first three minutes, but is very feeble at the end of half 

 an hour. These conditions are greatly modified by temperature, 

 and in a much less degree by the aeration of the ovum. Experi- 

 ments in proof of these facts are detailed, especially with reference 

 to the number of eggs segmented and of embryo produced, and their 

 earlier or later appearance in proportion to the higher or lower tem- 

 perature of the medium. In March 1849 the author put to the test 

 the agency of the spermatozoa in impregnation, by an experiment 

 long since employed by Spallanzani, and more recently by Prevost 

 and Dumas, namely, by carefully separating the spermatozoa of the 

 Frog from the liquor seminis by filtration, and employing these, 

 with the filter-paper on which they had been collected, in experi- 

 ments on some sets of eggs, and the liquor in others ; and the result 

 was, that almost every ovum became impregnated in the former, but 

 scarcely a single ovum in the latter. The production of a very few 

 embryos in the sets to which the liquor seminis was added, he 

 attributes to the fact that the whole of the spermatozoa had not 

 been removed. These experiments he has repeated during the pre- 

 sent spring with still more decided results ; not a single ovum be- 

 coming segmented, nor a single embryo produced when the liquor 

 seminis was completely freed of spermatozoa. The author states that 

 these experiments had been completed, and he v/as engaged in pre- 

 paring the paper for presentation to the Royal Society, before he 

 was aware that the physiologists above named, Spallanzani first, and 

 afterwards Prevost and Dumas, had obtained similar results by fil- 

 tration of frog's semen, although the fact of their observations has 

 been almost overlooked. To them therefore he resigns the credit 

 of the results ; but as his own investigations have been so com- 

 pletely independent of- theirs, from which also they difler in some 

 respects, he has felt it to be desirable still to give them in detail in 

 this paper. 



The direct agency of the spermatozoa in impregnation being 

 thus proved, the author proceeds to investigate its nature. He first 

 shows that the ova are not impregnated after the motive power in 

 the spermatozoa has ceased. The period of duration of this power 

 he finds to be much shorter than supposed by Spallanzani and by 

 Prevost and Dumas ; and he attributes the difference in length of 

 time as observed by these authors and himself, to their having 

 adopted the objectionable mode of procuring the seminal fluid by 

 vivisection from the testes as well as the vesicular seminales, by 

 which he conceives that spermatorial cells were obtained as well as 

 mature spermatozoa, and that the former became matured only at a 

 late period of the experiments. He differs also from Prevost and 

 Dumas, and Dr. Martin Barry, with regard to the supposed pene- 



