At the breeding season the sperm-ducts of the male are filled with
sperm, so much so that if they be squeezed the sperm flows out
spontaneously as a white fluid; the ducts are bifurcate, and start
from the midriff and the great vein. About this period the sperm-ducts
of the male are quite distinct (from the womb of the female) but at
any other than the actual breeding time their distinctness is not
obvious to a non-expert. The fact is that in certain fishes at certain
times these organs are imperceptible, as was stated regarding the
testicles of birds.

Among other distinctions observed between the thoric ducts and the
womb-ducts is the circumstance that the thoric ducts are attached to
the loins, while the womb-ducts move about freely and are attached
by a thin membrane. The particulars regarding the thoric ducts may
be studied by a reference to the diagrams in my treatise on Anatomy.

Cartilaginous fishes are capable of superfoetation, and their
period of gestation is six months at the longest. The so-called starry
dogfish bears young the most frequently; in other words it bears twice
a month. The breeding season is in the month of Maemacterion. The
dog-fish as a general rule bear twice in the year, with the
exception of the little dog-fish, which bears only once a year. Some
of them bring forth in the springtime. The rhine, or angel-fish, bears
its first brood in the springtime, and its second in the autumn, about
the winter setting of the Pleiads; the second brood is the stronger of
the two. The electric ray brings forth in the late autumn.