THE OVUM IN THE AMPHIBIA. 199 



has been gradually rising at the time of impregnation, the fecundation of the ovum, 

 as I have stated, has more certainly taken place than when the temperature was sub- 

 siding, the condition of the ova and of the impregnating fluid employed being equally 

 fit in each case. SPALLANZANI has shown that in his experiments ova did not become 

 impregnated after they had remained fifteen minutes in water. In the experiments 

 by myself I could rarely obtain fecundation after thirty minutes' immersion. The 

 difference of time between these results may fairly be attributed to difference in the 

 temperature at which the experiments were made, and in great measure to the in- 

 fluence of this on the endosmosis and expansion of the envelopes. But it was possible 

 that some other agent might be concerned in these results, and that light, as well as 

 heat and immersion in water, might greatly influence them. To put this to the test, 

 and to learn whether the difference depends entirely or chiefly on the amount of tempe- 

 rature, I have made two sets of experiments at precisely the same time, performed in 

 the same way, with ova from the same female and impregnating fluid from the same 

 male, the only difference being that within a very few minutes after the impregnating 

 fluid was supplied, one set was removed to a higher and slightly rising temperature, 

 from which all light was excluded ; while the other was allowed to remain freely ex- 

 posed to light, but in a room of ten or twelve degrees lower temperature, and which 

 was becoming still further reduced. 



The influence of light and heat on the development of the embryo has already 

 been referred to by SPALLANZANI, PREVOST and DUMAS, RUSCONI, Dr. W. EDWARDS, 

 and Mr. BELL. RUSCONI expressly states that light has no influence on the develop- 

 ment of the germ*, but his observations, as well as those before made by SPALLANZANI, 

 show that heat has a very marked influence, and this has been fully confirmed by 

 Dr. W. EDWARDS, and Professor BELL. Very recently also the subject has been 

 referred to by Mr. HIGGINBOTTOM-J-, and I have great pleasure in stating that my 

 own observations on the influence of heat, and the little effect of light on the develop- 

 ment of the tadpole, are in accordance with the observations made by him. But 

 the object I have had most in view has been, as above stated, to mark the effect of 

 heat, without light, on the changes of the ovum, more especially during the period 

 of fecundation, the first three or four hours after the egg is laid ; and onwards to the 

 termination of what I shall hereafter propose to consider, when 'describing the deve- 

 lopment of the embryo, as the end of the third period the closure of the lamina** 

 dorsales and the establishment of ciliary aeration on the surface of the body. 



Set H, March 20, 1850. Atmosphere 59 FAHR. Water 57 FAHR. 



No. 1. P.M. l h 14 m . Eighteen ova, as they passed from the body of a frog, were 

 touched lightly once with a hair-pencil that had been dipped in impregnating fluid 

 obtained two minutes before, and mixed with about three parts of water. After 

 these ova had remained ten minutes in water, this was removed and fresh supplied. 



These ova assumed the ovoid form at the expiration of three hours and thirty-six 



* Loc. cit., p. 20. t Proceedings of the Royal Society, May 16, 1850; and Phil. Trans. Part II., 1850. 



