Mesoblast

“ In the hindermost part of the embryo the mesoblast is at its maximum, and forms, on each side, a continuous sheet extending from the median line to the periphery (fig. 8a) . The rounder form of the mesoblast cells renders the line of junction between the layer constituted by them and the hypoblast fairly distinct; but towards the periphery, where the hypoblast cells have the same rounded form as the mesoblast, the fusion between the two layers is nearly complete. ”

“ The outer mesoblast, lying immediately under the epiblast, constitutes the substance of the somatopleur, and from it will be formed the dermis, the muscles of the body wall, almost all the cartilage and bone of the skeleton, the substance of the limbs, the kidneys, genital organs, heart and bloodvessels, and, in short, everything between the dermis and the coelom, except the nervous system and nerves, and the notochord. ”

“ A special mesoblast cell on each side elongates into two processes, the commencing limbs of the future organ. A lumen is developed in this cell, which is continued into each limb, while [Pg 682] the continuations of the two limbs are formed by perforated mesoblast cells.
According to Fol these organs originate in aquatic Pulmonata as a pair of invaginations of the epiblast, slightly behind the mouth. Each invagination grows in a dorsal direction, and after a time suddenly bends on itself, and grows ventralwards and forwards. ”

“ The thickened portions of mesoblast which caused the tail-swellings are still to be seen and would seem to act as the reserve from which is drawn the matter for the rapid growth of the tail, which occurs soon after this. The mass of the mesoblast at the base of the brain has again increased. No fresh features of interest are to be seen in the notochord. The heart is now much more conspicuous than before, and its commencing flexure is very apparent. It now beats actively. The hind gut especially is much longer than in the last specimen ”

“ In so excessively highly differentiated a type as birds we could hardly expect to find, and certainly do not find, any traces of the [Pg 35] primitive origin of the mesoblast, either from the epiblast or hypoblast, or from both. In the chick the mesoblast cells are formed directly from the ultimate products of segmentation. From having a secondary origin in most invertebrates the mesoblast comes to have, in the chick, a primary origin from the segmentation spheres, precisely as we find to be the case with the nervous layer in osseous fishes. ”

“ That the mesoblast and hypoblast are formed nearly synchronously, and except at the centre a fairly even sprinkling of lower layer cells is from the first to be distinguished between the epiblast and hypoblast. 2nd. That if some of the lower layer cells are not converted into mesoblast, it is difficult to see what becomes of them, since they appear to be too numerous to be converted into the hypoblast alone. ”

“ If embryology gives no certain sound as to the questions just raised with reference to the body cavity, still less is it to be hoped that the remaining questions with reference to the origin of the mesoblast can be satisfactorily answered. ”

“ There are at first but very few connective-tissue cells between the epithelium of the somatic layer of the mesoblast and the epiblast, but a connection between them is established by peculiar protoplasmic processes which pass from the one to the other (Pl. 11, fig. 8) . Towards the end of stage K, however, there appears between the two a network of mesoblastic cells connecting them together. In the rudimentary outgrowth to form the limbs the mesoblast cells of the somatic layer are crowded in an especially dense manner. ”

“ And from the mesoblast originate the bones, the muscles, the heart and blood-vessels, and the lymphatics, together with such parts of various internal organs as are most remotely concerned with the outer world. Minor qualifications being admitted, there remain the broad general facts, that out of that part of the external layer which remains permanently external, are developed all the structures which carry on intercourse with the medium and its contents, active and passive ”

“ The mode of formation of the mesoblast is not certainly known in Amphioxus; we shall find, however, that for all other vertebrates it arises from the cells which are homologous with the involuted cells of this animal.
Since food material is a term which will be very often employed, it will be well to explain exactly the sense in which it will be used. ”

“ This process goes on till all the cells are so filled by the mesoblast, with its myriad brood of cells, that the outer sac or ectoblast becomes a mere halo around it. Then every mesoblast contracts; the contraction deepens, till it is divided across in both directions, separating thus into four parts, then into eight, then into sixteen, and so on, till every cell is crowded with hundreds of minute mesoblasts, each containing the indication of a central dot or entoblast. At this period every yolk cell is itself like a whole yolk; for each cell is as full of lesser cells as the yolk-bag itself. ”

“ Moreover the lower layer cells outside the embryo now exhibit distinct traces of a separation into two layers, one continuous with the hypoblast, the other with the mesoblast. Both layers are composed of very flattened cells, and the mesoblast layer is often more than one cell deep, and sometimes exhibits a mesh-like arrangement of its elements. ”

“ The similarity of the mesoblast and nerve-tissue in Scyllium and Pristiurus embryos hardened in picric or chromic acid, render the nerves in these genera, at the stage when they first become enveloped in mesoblast, difficult objects to observe; but no similar difficulty is encountered in the case of Torpedo embryos.
While the rudiments of the posterior roots are still quite short, those of the anterior roots make their first appearance. Each of these (Pl. 14, fig. 4, a.r.) arises as a very small but distinct conical outgrowth from a ventral corner of the spinal cord. ”

“ Epiblast, hypoblast, and mesoblast are at first alike constituted of nothing but nucleated cells, and they increase in dimensions by the continual fission and growth of these cells. The several layers become gradually modelled into the organs which they constitute, before the cells undergo any notable modification into tissues. A limb, for example, is, at first, a mere cellular outgrowth, or bud, composed of an outer coat of epiblast with an inner core of mesoblast ”

“ In Mammals there appear to be some mesoblast cells invaginated with the lens, which are not improbably employed in the formation of the vessels of the so-called membrana capsulo-pupillaris. In the Ammocœte the vitreous humour originates from a distinct mesoblastic ingrowth, though the cells which give rise to it subsequently disappear. ”

“ I must also refrain from dwelling at length on the fact already adverted to, that the intermediate derived layer, or mesoblast, which was at the outset completely internal, originates those structures which ever remain completely internal, and have no communication with the environment save through the structures developed from the other two: an antithesis which has great significance. ”