Parxer.—On the Gravid Uterus of Mustelus antarcticus. 219 
Arr. XXVIII.— On the Gravid Uterus of Mustelus antarcticus. 
By T. Jerrery Parxzr, B.Sc. 
[Read before the Otago Institute, 31st October, 1882.] 
Plate XXX 
Tue viviparous dog-fish Mustelus is remarkable for the fact that in one of its 
species, M. levis of the northern seas, a vascular connection is established 
between the foetus and the mother by the yolk-sac of the former entering 
into close contact with the wall of the uterus, and thus forming an ** um- 
bilical placenta.” This arrangement becomes all the more remarkable from 
the circumstance that in the other species of the genus no such connection 
obtains. 
In Günther's ** Catalogue of Fishes," as well as in his more recent 
work, “ The Study of Fishes,” the common southern Mustelus, M. ant- 
arcticus, is merely said to be, like M. vulgaris and other northern forms, 
devoid of an umbilical placenta, from which one would naturally expect to 
find the foetuses lying freely in the uterine cavity, as in other viviparous 
sharks—e.g., Scymnus or Acanthias. I was therefore considerably surprised 
to find, on dissecting a gravid female of M. antarcticus a week or two since, 
that the relations between the mother and the foetus were nothing like so 
simple as I had expected, but that, just as the Mustelus levis furnishes a 
sort of foreshadowing of the true placenta of mammals, so M. antarcticus is 
provided with membranes which, although formed from the maternal and 
not from the fotal tissues, foreshadow in a remarkable manner the chorion’ 
and the amnion. 
The specimen referred to, and others dissected subsequently, were evi- 
dently near delivery, since the foetuses (see fig. 1) were large and perfectly 
formed, and their yolk-sacs (yk. s) were reduced to the size of a small pea. 
On opening the abdomen the uteri were at once noticeable from their great 
transparency and extreme tenseness: the fetuses could be plainly seen 
through their walls, and the uteri themselves had the appearance of being 
distended with fluid. By squeezing the uterus from the outside each fetus 
could be only very slightly displaced ; it was evident that they were con- 
fined in some way, but not by actual attachment to the uterus. 
The explanation of these appearances was at once evident on opening the 
uterus. Each foetus was then seen to be enclosed in a separate compart- 
ment, filled with a colourless fluid, in which it floated freely. The partition 
walls between adjacent compartments are evidently quite impervious, so 
that there was no communication between them, nor between the anterior 
compartment and the cavity of the Fallopian tube (fig. 1, /'/.t.) or the posterior 
compartment and the cloaca. 
