FRO 
FRO 
month of April; but it generally begins to ope- 
rate about the middle of March. The male is 
ufually of a greyifh brown colour; but the female 
is more inclining to a yellowilli hue, and fpotted 
with brown. When they copulate, the colours of 
both are nearly alike on the back; but, as they 
change their fkins almofb every eight days, the 
old ones falling off in the form of a mucus, the 
male becomes more yellow, and the female more 
brown. The arms and legs of the males are much 
ftronger than thofe of the females; and, at the 
time of impregnation, they have a kind of flefhy 
excrefcence on their thumbs, which they firmly fix 
to the breafts of the females. This LinnjEus fup- 
pofed to be the male inftrument of generation; 
but, on a minute infpeftion, it is found only con- 
ducive to keep the female in a more ftrift embrace: 
it may be amputated, and the impregnation conti- 
nue unimpaired ; and it is even fometimes found 
in the females, while fome of the other fex are en- 
tirely deftitute of it. 
The fexcs couple only once a year, and then 
they continue united fometimes for four days fuc- 
ceffively. At this time tlieir bellies are greatly in- 
flated ; that of the female is filled with eggs ; and 
the fkin of the male is diftended with a kind of 
limpid water, which is ejefled in impregnation. 
As foon as the male has leaped on the female, he 
throws his fore-legs round her bread, and clofcs 
them fo firmly, that they are with no fmall diffi- 
culty difengaged. This grafp appears to be in- 
voluntary and convulfive; the animals cannot be 
torn afunder without a laceration of the parts; they 
fwim, creep, and leap, thus united, till the female 
has fhed her fpawn, which at length fiie performs 
almoft in an inftantaneous manner. How the im- 
pregnation is performed without any apparent in- 
ftruments of generation, has long been an objed: of 
enquiry, and ftill continues undetermined. In 
order to folve this difficulty, Rsfel, of Nuremberg, 
perfevered in an examination of their mutual con- 
grefs for three years fucceffively ; and availed him- 
ielf of all the lights which difleflion, or the analogy 
of formation, could furnifh him. Having chofen 
twelve couple of Frogs thus united to each other, 
and placed each couple in a glafs veflfel filled with 
water, he kept them in his view almoft day and 
night, and even fat up two nights together for the 
purpofe of examining their operations. The firft 
day, he obferved nothing that deferved remark; 
the fecond, they began to be more agitated, the 
males making a noife fomewhat refembling the 
grunting of hogs; while the females only conti- 
nued to fink and rife alternately in the water. The 
male of the firft couple ejeded the humidity with 
which his body was fwoHen, and foon after quit- 
ted the female. Our philofopher continued for 
twelve hours to obfcrve whether the female would 
emit her fpawn ; but finding that flie was tardy, 
he difilided both her and the male. In the latter, 
the fpermacetic veflels were entirely empty, as 
might have naturally been fuppofed; but the fpawn 
of the female ftill continued in her body: on it's 
being extrafted and put into water, it perifhed 
without producing any animal whatever; hence he 
rationally concluded it requifite that the eggs 
fliould be ejefted from the body of the female be- 
fore they could be prolific. In another pair, the 
male quitted the female, which did not ejed her 
fpawn till fixteen days after; and thefe, like the 
former, came to nothing. But, with refped to 
fome of the reft, the cafe was very different : the 
females ejefled their fpawn; while the males re- 
mained in their ftations, and impregnated the 
mafles, at different intervals, as they fell from tiie 
females; and thefe all produced animals in the 
ufual courfe of generation. From thele obferva- 
tions, it is eafy to infer, that the female was neither 
impregnated by the mouth, as fome philofophers 
have imagined, nor by the excrefcences at the 
thumbs, as was the opinion of Linnjeus ; but by the 
infperfion of the male feminal fluid on the eggs of 
the female, as they proceeded from her body. 
A recent publication, by that very penetratinff 
naturaliff the Abbe Spallanzani, feems, indeed, to 
throw all the light on this fubjeil which can ever 
be expefted. This gentleman made the fame ex- 
periments as Rtefel, and found the fame effefts: of 
one hundred and fifty-fix females v/hich he opened, 
after they had been fome time in the em.braces of 
the males, but before they had made any volun- 
tary emiffions, he did not find one egg which was 
prolific; whereas thofe excluded fpontaneoufly by 
the females were all endowed with the vital prin- 
ciple. He, however, farther obferved, that as foon 
as the eggs began to be difcharged, the agitations 
of both the male and the female v/ere extreme; and 
that an obtufe point, Vs^hich he fufpefted to be the 
penis, was elongated, and occafionally brought to- 
wards the eggs neareft the vent; but he was un- 
able to perceive any emiffion. In order to clear up 
this point, he placed fome couples, of which the 
females were beginning to difcharge their eggs, in 
empty veflels. He fucceeded in this experiment 
beyond expe<ftation ; for fuch v/as the attachment 
of the males, that they perfifted in the performance 
of their office, though taken out of their natural 
element. The Abbe now clearly perceived a fmall 
jet of limpid liquor falling from the tumid point 
in the vicinity of the anus upon the eggs extruded 
from the body of the female. The eggs being 
afterwards put into water, and bringing forth 
young, he concluded, without hefitation, that the 
liquor was real femen; and was afterwards juftified 
in this opinion, by difcovering it in the veficulae 
feminales, as well as by a whimfical and feemingly 
ludicrous experiment of putting breeches of waxed 
taffety on the male; when, notwithftanding this 
incumbrance, he fought the female with equal ar- 
dour: but the event was fuch as he might natu- 
rally have expected ; the eggs were never prolific, 
and the femen appeared in the breeches in the 
fhape of drops; and that thefe drops were real fe- 
men, was afcertained beyond all difpute, by an ar- 
tificial fecundation which the Abbe obtained by 
their means. 
A fingle female Frog produces from fix to ele- 
ven hundred eggs at a time; and, in general, flie 
throws them all out together by one effort; though 
in this operation fhe is fometimes occupied for the 
fpace of a whole hour. While thus employed, 
the male may be obferved performing the part of 
a midwife in promoting the expulfion of the eggs, 
by working with his thumbs, and compreffing the 
body of the female. The eggs being emitted, ex- 
pand themfeives into a round form, and drop to 
the bottom of the water; while the male fwims off, 
and ftrikes his arms with his ufual freedom an4 
agility. The egg, or little black globe, which 
produces the tadpole, is encompafted with two 
different kinds of fluid: that which immediately 
furrounds the globe is clear and tranfparent, and 
inclofed in it's proper membrane; while that 
which furrounds the whole is muddy and mucous. 
The 
