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Scientific Memoirs by

      The placenta, formed by the yolk-sack, is a compact arborescent mass
adherent to a vascular disk on the uterine wall near its after end: its cord is
about 18 inches long. Its blood-vessels are the ramifications of a single artery
and a single vein: the artery takes off from a glandular pouch at the level of the
duodenum, but its connexion with the dorsal aorta was not made out; the vein
passes by two branches into the liver.

      A female of Carcharias dussumieri, 7½ feet long, taken in the month of
April, contained ten young, each two feet long—five in either oviduct.

      The arrangements differ from those of Carcharias melanopterus only in the
one particular that the artery conveying the blood to the yolk-sack placenta had
no glandular dilatation near its point of origin.

      A female of Zygœna blochii nearly five feet long, taken in the month of
January, contained ten young, each about 15 inches long—five in either oviduct.

      The arrangements as regards the packing and envelopment of the fœtus,
and as regards the formation and vascularization of the yolk-sack placenta do not
differ from those of Carcharias melanopterus, except that the umbilical artery
has no gland near its point of origin, and that the greater part of the placental
cord is festooned with leaf-like villi, as in Zygœna tiburo and in a species of
Carcharias described by Leuckart and Cuvier.

      These villi are very characteristic. Each consists of (1) a core of delicate
connective-tissue supporting a plexus of blood-vessels and masses of cells in its
meshes, and (2) an envelope formed of numerous layers of round or cuboid large-
nucleated cells. Their vascularity and their richly cellular structure show that
they must be the seat of active metabolism, but as there is also a large placenta,
their necessity is not immediately apparent.

      I may add that I have verified my original observations on this species from
specimens since brought to the Museum by local fishermen.

      Of the Elasmobranchs that nourish the fœtus on a uterine milk, which the
fœtus swallows and digests, the " Investigator " has discovered certainly five
species—namely, Trygon bleekeri, walga, and zugei, Pteroplatœa micrura, and
Myliobatis nieuhoffii—and probably a sixth, namely Narcine timlei.

      In all, the process is, in essence, the same. The embryo at first lives on
the copious yolk of the egg, and when the yolk is all absorbed (a process which,
in Pteroplatœa micrura at any rate, is effected not by any vascularization of the
yolk-sack, but, in part at least, by special vascular external gill-filaments) the
yolk-sack, instead of forming a placenta, simply shrivels up.

      No enveloping membranes of any sort have been formed, so that the fœtus
lies quite naked in the uterine cavity.