Dr. James Blake read the following communication:
On the Nourishment of the Fœtus in the Embiotocoid Fishes.
BY JAMES BLAKE, M.D., F.R.C.S.
I am not aware that the process by which the embryo of the Embiotocoid fishes receive the nourishment necessary for its growth, has ever been pointed out. It certainly differs from the three most common forms in which the embryo of other animals is nourished, as there is nothing like a placenta by which they can receive nourishment from the mother; there is no supply of nutriment surrounding the embryo, as in the case of most oviparous animals, nor is the embryo brought into direct contact with the water, so as to derive nourishment by absorption from the surrounding medium, as is the case in oviparous fishes generally and in most of the lower forms of animal life. The young fish is contained in a uterus which, in the undeveloped state, resembles very much the ovaries of the common oviparous fishes, except that its walls are thicker, and that the number of ova it contains is very much smaller. In the interior of the uterus, projecting from its sides, are a number of processes analogous to those to which the ova are usually attached. These processes vary in number in different examples, but they are so arranged that each fœtal fish is in contact on every side with a surface of one of these processes. They consist apparently of a membrane composed of a cellular tissue, and scattered over their surface are a number of small mammillary elevations with an orifice in the center, and which are probably the organs by which the peculiar secretion of the uterus, to be hereafter noticed, is poured out. In an example I examined, in which impregnation had apparently just taken place, numerous ova were found adhering to these processes, although not at all in such numbers as in the ordinary fishes. I counted thirty-eight in about the space of an inch; of these, however, but few can be developed, as the number of fœtuses seldom exceeds forty, and sometimes is only eight. In the whole of the uterus there probably were from one hundred to one hundred and fifty ova. Of the earlier stages of development, however, it is not my object to treat in the present memoir, as I did not commence my investigations sufficiently early to be able fully to make it out. As soon, however, as the embryo has advanced sufficiently for the fins to be formed, these appendages are found to be terminated by a number of digitations, which project from the free edges of the fin, and are usually found situated, one between each ray or spine. They are composed almost entirely of fine capillary blood-vessels, united apparently by a very delicate and structureless membrane. They are so delicate that unless great care is taken in removing the specimen from the uterus, they are destroyed; nor have I ever been able to discover them in specimens that have been preserved in alcohol. These processes seem continuous with the membrane extended between the rays of the fins, but are much more delicate; they project from the free edge of the fin, sometimes as much as the eighth of an inch, and are, in the fully developed embryo, the fifteenth of an inch broad. On the free margin of each digitation, a larger capillary can be observed, which appears to be continuous all around; it is about the .003 in. in diameter, the intermediate space being filled with a net-work of smaller capillaries. This system of digitations projects from the entire edge of the dorsal, ventral and caudal fins, but not from the pectorals. They in fact form a fringe round the entire body, with the exception of the head and that part of the abdomen in front of the anus.
Such is the structure of the organ that evidently has some connection with the nourishment of the fœtus, resembling as it does so closely the early formation of the vascular villi and the placental tufts that proceed from the chorion of the mammiferous embryo, and through which it derives its nourishment before the placenta is fully formed.
The question now presents itself as to how nourishment is conveyed from the parent to the fœtus through these tufts? As before stated, the lining membrane of the uterus sends off processes which surround each fœtus, without however forming sheet sacks; but although these processes are very freely supplied with blood-vessels, yet the finest injection failed to show any more vascular spots where the fœtal digitations might have been brought into more immediate contact with the blood of the parent. I however was fortunate enough to obtain a fish, in the uterus of which I discovered a considerable quantity of fluid, and on collecting it, and submitting it to chemical tests, I found that this fluid contained a considerable quantity of an animal substance, resembling, to a certain extent, some of the compounds that are formed from albumen during the process of digestion. The fluid was of yellowish color, translucent, deposited on standing some small globules which under the microscope strongly refracted the light, were not altered by acetic acid, but dissolved in ether; probably fat globules; when heated, there was no coagulation, although the fluid was not quite so clear; solution of Hg Cl₂ caused no precipitate; tannin in solution caused a yellowish precipitate. In adding ether to a portion of the fluid, there was a free disengagement of gas, a white flocculent precipitate was formed, and on allowing the vessel to stand, the fluid separated itself into three portions: the upper portion consisting of pure ether apparently, then a layer containing white flocculi, which occupied about the fourth part of the fluid, and below this the remains of the original fluid, but little altered in appearance. There can, I think, be little doubt but that it is through the medium of this fluid that the fœtus obtains its nourishment. The considerable portion of animal matter it contains, and that too in a state particularly fitted for absorption and for conversion into tissue, fits it for furnishing the fœtus with the elements necessary for its growth by absorption through the large surface of capillary vessels which are found in the vascular digitations that surround the fœtus, and which are constantly bathed in the fluid. The difficulty that up to the present time has attended every attempt to trace the connection between the parent and fœtus in these embiotocoid fishes, is owing, in the first place, to the extreme delicacy of the vascular digitations of the fœtus, which prevents their being observed in preserved specimens, and also to the fact that in almost every case the fluid secreted by the uterus is entirely expelled by the violent struggles of the fish when removed from the water, so that it was almost by a rare accident that I succeeded in obtaining any. I hope, however, during the coming season, to be able more fully to carry out these researches.
Fig. 30. A Fœtal Fish, about two-thirds grown, slightly enlarged.
Fig. 31. A portion of Dorsal Fin of an almost mature fœtal fish, about double the natural size.