of the generative season, in all the lower animals,
a certain number of the eggs, which were previously in an imperfect
and inactive condition, begin to increase in size and become somewhat
altered in structure."

"In most fish and reptiles as well as in birds, this regular process
of maturation and discharge of eggs takes place but once in a year.
In different species of quadrupeds it may take place annually,
semi-annually, bi-monthly, or even monthly; but in every instance it
recurs at regular intervals, and exhibits accordingly, in a marked
degree, the periodic character which we have seen to belong to most
of the other vital phenomena."

"In most of the lower orders of animals there is a periodical
development of the testicles in the male, corresponding in time with
that of the ovaries in the female. As the ovaries enlarge and the eggs
ripen in the one sex, so in the other the testicles increase in size,
as the season of reproduction approaches, and become turgid with
spermatozoa. The accessory organs of generation, at the same time,
share the unusual activity of the testicles, and become increased in
vascularity and ready to perform their part in the reproductive
function."

"Each of the two sexes is then at the same time under the influence
of a corresponding excitement. The unusual development of the genital
organs reacts upon the entire system, and produces a state of peculiar
activity and excitability, known as the condition of 'erethism.'"

"It is a remarkable fact, in this connection, that the female of these
animals will allow the approaches of the male only during and
immediately after the oestral period; that is, just when the egg is
recently discharged, and ready for impregnation. At other times, when
sexual intercourse would be necessarily fruitless, the instinct of the
animal leads her to avoid it; and the concourse of the sexes is
accordingly made to correspond in time with the maturity of the egg
and its aptitude for fecundation."

"The egg, immediately upon its discharge from the ovary, is ready for
impregnation. If sexual intercourse happens to take place about that
time, the egg and the spermatic fluid meet in some part of the female
generative passages, and fecundation is accomplished.... If, on the
other hand, coitus do not take place, the egg passes down to the uterus
unimpregnated, loses its vitality after a short time, and is finally
carried away with the uterine secretions."

"It is