merely the increasing distension of the uterus by the developing foetus?
This could scarcely be the case in the Marsupials in which the foetus when
born is quite minute. Nor can we attribute parturition to renewed
ovulation, for this occurs in _Dasyurus_ only once a year. All we can
suggest at present is that a certain periodic development takes place by
heredity in presence of the hormones exuded by the fertilised ovum and the
embryo developed from it. When the ovum or ova, not being fertilised, die
the period of development is (usually) shortened and pro-oestrus or
menstruation occurs. In the dog, however, the period of the oestrus cycle
is about the as that of gestation--namely, six months.

The so-called descent of the testicles occurs exclusively in Mammals, in
which with a few important exceptions it is universal. This is a very
remarkable case of the change of position of an organ in the course of
development. The original position of the testis on either side is quite
similar to that of the same organ in birds or reptiles. The genital ridge
runs along the inner edge of the mesonephros, with which the testicular
tubules become connected. The testis, with the mesonephros, forming the
epididymis, closely attached to it, projects into the coelom, and without
losing its connexion with the peritoneum changes its position gradually
during development, passing backwards and downwards until it comes to lie
over the wall of the abdomen just in front of the pubic symphysis of the
pelvic girdle. There the abdominal wall on either side of the middle line
becomes thin and distended to form a pouch, the scrotal sac, into which
the testis passes, still remaining attached to the peritoneum which lines
the pouch, while the distal end of the vas deferens retains its original
connexion with the urethra. The movement of the testis can thus be
accurately described as a transposition or dislocation.

Various causes have been suggested for the formation of the scrotum, but