THE OVUM IN THE AMPHIBIA. 
203 
successful than what takes place with regard to the ova in the natural haunts of the 
species. The ova in a state of nature are usually deposited in well-aerated places, 
clear, slow-moving water, or shallow and but slightly turbid water. It is almost 
impossible to afford to ova that are the subjects of experiment, either in broad flat 
dishes, or in glass vessels in one’s study, the amount of aeration required to ensure 
complete success. By too frequently changing the water in the vessels the embryos 
often become injured ; while if the water be not changed, development is arrested, 
and decomposition commences, and the experiment entirely fails. Even when these 
difficulties are obviated by a gentle withdrawal of the water, and a renewal of it with 
equal care, the perfect stillness of the fluid in the interval of our observations does not 
allow of that extent of aeration to the embryo which it gains in a perfectly natural 
state, either in slow-moving waters, where I have usually found the eggs deposited, 
or in pools of still water, the surface of which is agitated by currents of air, and 
affected by diurnal changes of temperature. 
Thus then we may conclude that the procreative force of the germ, and of the im- 
pregnating fluid, is augmented by increase of heat, but the duration of the force is 
lessened. It becomes less and less energetic in proportion as the temperature is dimi- 
nished, but the period during which it is capable of being exerted is extended. In 
each of these conditions aeration is of essential consequence, and becomes more and 
more necessary in proportion to the increase of heat. 
4. THE AGENCY OF THE SPERMATOZOA IN IMPREGNATION. 
It is evident from the last-mentioned experiments, that however great may be the 
influence of temperature in accelerating or retarding impregnation and develop- 
ment, and however much the operation of this influence may be interfered with by 
want of proper aeration of the ovum, the impregnating force does not equally pertain 
to all parts of the seminal fluid, but is to be found in some only of its constituents. 
Experiments made before those now detailed, — and to which I had been led by a 
convietion that the opinion formerly entertained, that impregnation is effected 
through means of the fluid portion only of the semen, was not in accordance with 
facts I had very long been acquainted with in the Articulata, — convinced me that 
the spermatozoa themselves, and not the other constituents of the semen, are the 
efficient agents of impregnation. Leewenhoek, and, as I have since found, Prevost 
and Dumas, not only believed this to be the fact, but also held the opinion that the 
spermatozoa penetrate bodily into the ovum ; and this view has been more recently 
insisted on by Dr. Martin Barry, with the additional belief that a perforation or fissure 
exists in the envelopes of the ovum, through which the spermatozoon enters. On care- 
ful examination of the envelopes of the ovum of the Frog, I have not been able to 
detect any fissure or orifice. The question of the agency of the spermatozoa, never- 
theless, appears to be capable of solution, however difficult it may be to ascertain the 
mode and particular nature of such agency. The separation, as far as possible, of 
2 D 2 
