Medical Officers of the Army of India.

57

7. On Viviparous Elasmobranch Fishes.

      As is well known, the impregnation of the ovum in this group is always
effected internally. In some Elasmobranchs, such as the Dog-fishes, the true
Rays, and the Chimæras, the female lays large eggs, enclosed each in a tough
horny envelope which is often of fantastic shape or curiously ornamented.

      In others—e.g ., according to T. J. Parker, in Mustelus antarcticus— the
embryo develops in the uterus, which is merely a membranous bag having no
special structures for supplying the embryo with nutriment, and not being in any
vascular contact with the yolk-sack; so that though the fœtus is nourished by
the mother, the exact steps of the process are still unknown.

      In others, including a large number of species of sharks, the embryo develops
in the uterus, and when all the yolk is consumed the vascular yolk-sack
becomes applied to the vascular uterine wall to form a yolk-sack placenta.

      In others, as in several species of Sting-rays, Eagle-rays, and Torpedoes,
the embryo completes its development within the uterus, and is nourished—as the
observations made on board the " Investigator " prove beyond dispute—not by a
placenta, but by what may, without laxity of language, be called a kind of milk,
secreted by special glands of the uterine mucous membrane.

      Of the eggs of the oviparous Elasmobranchs the " Investigator " has had
little experience. Empty egg-cases, similar to those of Callorhynchus and
Chimœra, have been dredged in the Andaman Sea and the Bay of Bengal.

      Again, off the Travancore Coast, at a depth of 824 fathoms, a developing
egg that belongs, probably, to some gigantic species of Ray was dredged. This
egg is shaped just like the ordinary " Mermaid's purse " of British seas, and the
shell, measured without the filamentous appendages, is over 6½ inches long and
more than 4¾ inches broad. It contains a little embryo in what may be called
the Selachian stage, that is, the stage in which the pectoral fins are of no great
size and have not pushed the gill-clefts out of their lateral position.

      Of Viviparous Elasmobranchs in which the young is nourished by a yolk-
sack placenta, three species have come into our hands, namely Carcharias
melanopterus, Carcharias dussumieri, and Zygœna blochii.

      A female of Carcharias melanopterus, five feet long, taken in the month of
January, contained six young, each a foot long,—three in either oviduct.

      Each young one lay, head forwards, in its own separate compartment of the
uterus, in which, further, it was completely enveloped in a very delicate membrane
of its own. This delicate envelope is evidently the pseudamnion of Parker, which,
according to that author, corresponds with the horny egg-shell of oviparons
Elasmobranchs.