Art. XXVIII.—On the Gravid Uterus of Mustelus antarcticus.
[Read before the Otago Institute, 31st October, 1882.]
The 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 fœtus and the mother by the yolk-sac of the former entering into close contact with the wall of the uterus, and thus forming an “umbilical 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. antarcticus, 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 fœtuses 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 fœtus 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 fœtal tissues, foreshadow in a remarkable manner the chorion and the amnion.
The specimen referred to, and others dissected subsequently, were evidently near delivery, since the fœtuses (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 fœtuses 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 fœtus could be only very slightly displaced; it was evident that they were confined 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 fœtus was then seen to be enclosed in a separate compartment, 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,fl.t.) or the posterior compartment and the cloaca.
Each fœtus lay coiled up in its compartment (fig. 1), some part of its body, in many cases, pushing one of the partition walls and causing it to bulge out into the adjacent compartment. In the specimen figured, for instance, the head of fœtus h encroaches upon compartment g; the trunk of g encroaches upon compartment h, and its head upon f. The fœtuses are thus packed as closely as if they were not enclosed in separate chambers.
In the specimen figured there were eight fœtuses in the uterus, but the usual number seems to be five. In some cases one fœtus was considerably less developed than the rest: this is the case with a in fig. 1: in one instance there was in the anterior end of the oviduct a mass of yolk, evidently an egg which had undergone no development.
The precise anatomical relations are as follows:—The wall of the uterus, as mentioned above, is very thin: it consists of an outer peritoneal investment (fig. 2, p), then of a remarkably thin muscular layer (m), and finally of the mucous membrane (m.m.). The latter is produced into a series of reduplications which extend across the cavity to the opposite wall, and in this way the fœtal compartments are formed.
From this it is evident that the outer walls of the fœtal compartments are simply portions of the uterine walls, and are lined with epithelium, but that the party walls (ps. ch.) consist of mucous membrane only, covered with epithelium on both sides. The mucous membrane has a yellowish colour, is raised on its free surface into numerous folds, and is abundantly supplied with blood-vessels, so that each fœtus is surrounded with a vascular membrane.
From the inner surface of the mucous membrane, a thin colourless transparent non-vascular layer (fig. 2, ps. am.) can be readily dissected off. From the relations of the mucous membrane, as just described, it follows that this non-vascular membrane must occur in the form of a series of closed sacs, forming the actual lining of the several compartments.
As a consequence of this arrangement, when the peritoneal and muscular layers of the uterus are stripped off—which can be done with great ease—and the Fallopian tube and cloacal end of the uterus removed, the mucous membrane of the uterus proper is obtained in the form of a single perfectly closed sac, but on removing the mucous membrane itself, a number of closed sacs are obtained, each enclosing a fœtus with the surrounding fluid, and consisting of the non-vascular membrane just described.
It will be seen at once that the transparent non-vascular sac in which each fœtus is directly enclosed, has the same general relation to the fœtus as the amnion of Sauropsida and Mammalia, from which it differs in being a product, not of the fœtal but of the maternal tissues. I propose, there-
fore, to call it the pseud-amnion.* The fluid it contains and in which the fœtus floats is evidently a serous fluid, and, having the same relations to the fœtus as the amniotic fluid of the higher Vertebrata, may be called the pseud-amniotic fluid.
The outer or vasculan layer of each compartment, formed by the mucous membrane proper, contains the blood-vessels from which the fœtus derives its supply of oxygen: it is therefore roughly analogous to the chorion of mammals, and may be called the pseudo-chorion.
As regards the histology of the membranes, the most important fact is that the pseud-amnion is a true cuticle: it is quite structureless, and is in close contact with the free surface of the mucous membrane, from the epithelium of which it is evidently formed as a cuticular secretion.
As all the specimens I have hitherto examined have been in approximately the same stage of pregnancy, I have been unable to make any observations on the mode of formation of these remarkable membranes: one would be disposed to think, however, from their final disposition, that the investment of each impregnated ovum is formed in much the same way as the human decidua reflexa.
The pseud-amniotic fluid is colourless, transparent, and very slightly opalescent. Treated with nitric acid, it gives no trace of the xanthoproteic reaction, and may therefore be assumed to contain not more than the merest trace of proteids. Boiled with nitric acid, it gradually assumes a very darkbrown colour. Evaporated to a small bulk and treated with nitric acid, it gives an abundant crop of crystals of urea nitrata, so that it must contain a considerable quantity of urea, indicating an active renal secretion on the part of the fœtus. Evaporated to a third of its bulk and treated with hydrochloric acid, it assumes a pink colour, which gradually deepens into dark brownish-red, and deposits a fine pulverulent precipitate of a deep brown colour. Examined under the microscope this deposit shows no trace of uric acid crystal, and appears to consist entirely of fine amorphous granules. I can form no conjecture as to its nature.
From the arrangement of the fœtal membrane of Mustelus antarcticus, it is certain that both pseud-amnion and pseudo-chorion are ruptured at birth, and from this condition at the end of pregnancy, I feel sure that they are entirely thrown off,—in other words that they form a true decidua. From the extraordinary thinness of the muscular layer of the uterus, it is certain that it can be of little or no use in the expulsion of the fœtuses: it would seem that they must simply swim into the world, birth being due to the activity not of the mothers but of the offspring.
[Footnote] * There is hardly likely to be any confusion between this pseud-amnion and the so-called “false amnion” of Amniota.
Explanation of Plate XXX.
Fig. 1. Uterus of Mustelus antarcticus, with the several fœtal compartments opened from the ventral side (¾ nat. size).
|A—H, the eight fœtal compartments, with their contained fœtuses.||br, branchial aperture.|
|c, caudal fin.|
|e.f, partition between compartments E and F, pushed into F by tail of fœtus E.||d. 1, d. 2, first and second dorsal fins.|
|f.g, partition between F and G, pushed into F by head of G.||e, eye.|
|fl.t, Fallopian tube.|
|g.h, partition between G and H, pushed into G by head of H.||m, mouth.|
|g'.h', partition between G and H, pushed into H by trunk of G.||pc, pectoral fin.|
|pv, pelvic fin.|
|yk. s, yolk-sac.|
Fig. 2. Diagrammatic vertical section of the same to show the relations of the fœtal membranes to the uterine walls.
|A—D, four fœtal compartments.|
|fl. t, Fallopian tube.||ps. am, pseud-amnion.|
|m, muscular layer.||ps. ch, pseudo-chorion.|
|m. m, mucous membrane.|
|p, peritoneal investment.|