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'I-, 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 etfect 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 lamince 
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. I*' 14"*. — 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 Societ)’, May 16, 1850; and Phil. Trans. Part II., 1850. 
