' AND ON THE DIRECT AGENCY OF THE SPERMATOZOON. 239 



3. OF THE OVUM. 



The vitality of the eggs of the Frog appears to be of longer duration than that of 

 the fertilizing agent, the spermatozoon. The egg, as formerly shown*, leaves the 

 ovary quickly after, or at the time when its germinal vesicle disappears ; but, as 

 experiment has proved, it is not then entirely fitted for its function. It has yet to be 

 enveloped in a thick covering, which it gains in its transit through the oviduct, 

 before it is susceptible of impregnation. Its condition when escaping from the 

 ovary to the oviduct seems to be analogous to that of the spermatozoon when it 

 leaves the testis to enter the efferential duct. Neither the one nor the other 

 has yet attained its full maturity. When the eggs are collected in the dilated or 

 uterine portions of the oviducts they seem to be in a condition parallel to that of the 

 spermatozoa in the lower portion of the efferential ducts and vesicles. When nearly 

 the whole of the eggs are collected in the uterine cavities their maturity is then 

 almost completed ; but a period of retention, even in these structures, seems to be 

 necessary to ensure the fertilization of the whole, as some of them, as I have 

 found by the persistence of the germinal vesicle for a short time, even after the egg 

 has escaped from the ovary into the cavity of the abdomen, as the undeveloped 

 spermatozoal cell passes from the testis into the efferential duct, are later in their 

 development than others. Further, it may be remembered -}-, that so far from the 

 contents of the yelk being in a dormant condition at the time of oviposition, as some 

 inquirers have supposed;}:, there are changes still going on within it, and which are 

 perceptible to the eye, in the condition of the white surface, for ten or twelve minutes 

 after oviposition ; after which they become less and less marked, and soon entirely 

 cease if the egg be not fecundated. 



Thus then from a comparison of the state of the fecundatory agent with the body 

 to be fecundated, we might have expected to have found some close coincidence in 

 the retention of their vitality, and this indeed, in a state of nature, seems to be the fact. 

 Spawning seldom or never takes place by the act of the species until each sex has 

 attained its full maturity of function. The eggs are sometimes voluntarily retained 

 by the female Frog, for several hours or days within the uteri, although arrived at 

 maturity, if the proper evolution of spermatozoa in the male be not completed. I 

 have had evidence of this in the fact, that when the union of the sexes has been of 

 long continuance, the female has sometimes passed a very few ova, and then has con- 

 tinued without further oviposition for many hours, sometimes for a day or two, 

 before the mass was expelled, and the function of the sexes consummated. These first 

 extruded eggs have almost always been found to be unimpregnated. 



When the eggs have been deposited, the circumstances which affect their fecunda- 

 tion are precisely similar to those which affect the spermatozoa, the temperature 

 of the surrounding medium, and the length of time during which they remain im- 



* Philosophical Transactions, 1851, p. 180. f Loc. cit. p. 185. J WAGNER and LEUCKARDT, loc. cit. 



