Mesoderm

“ Between ectoderm and endoderm a third intermediate cell-layer is formed, which is called the mesoderm, and gives rise to the muscular and connective tissues to the vascular system, and to the excretory and generative organs. The mesoderm arises for the most part from the endoderm. When the segmentation is unequal one of the megameres gives rise by successive divisions to two primary mesoderm cells called mesomeres; these divide to form two masses of cells called mesoblastic bands. The coelom is formed as a cavity or cavities in the interior of these cell-masses. ”

“ Owing to the fact that the embryo may undergo considerable development before the egg is laid, and also to the unusual difficulty of removing the very young embryos, the earlier stages of development are very difficult to obtain.
The mesoderm seems to be derived chiefly by proliferation from the entoderm, in which way all of that anterior to the blastopore arises. Posterior to the blastopore the mesoderm is proliferated from the lower side of the ectoderm in the usual way. No distinction can be made between the mesoderm derived from the ectoderm and that derived from the entoderm. ”

“ These genital cells, which are transformed normal mesodermal cells, lie originally within the mesoderm layer or on the surface of this layer turned towards the yolk, on the edge of the segments. After the complete formation of the cœlom-sacs we find them (Fig. 549, gz) in the dissepiments which separate the successive cœlom-sacs from one another. Here new genital cells are constantly formed through the transformation of mesoderm cells. The development of the genital cells takes place in the 2d to the 7th abdominal segments. ”

“ It is enough for us that they always end in the formation of the two primary layers of ectoderm and endoderm.
The next stage of differentiation is common to all the Metazoa, except those lowest forms which, as we [142] Have just seen, remain permanently as large gastrulæ, with sundry specialized additions in the way of tentacles, &c. This stage of differentiation consists in the formation of either a pouch or an additional layer between the ectoderm and the endoderm, which is called the mesoderm. ”

“ Coelomic Organs.—The development of the mesoderm in the restricted sense (mesothelium) as seen in the fishes (lamprey, Lepidosiren, Protopterus, Polypterus) appears to indicate beyond doubt that the mesoderm segments of vertebrates are really enterocoelic pouches in which the development of the lumen is delayed. ”

“ The mesoderm is, if we may judge from the number of organs which are derived from it, much the most important of the three layers. It generally arises later than the others, and in its very origin presents difficulties to the theory, which are much increased when we consider its history. It is generally, though not always, developed from the endoderm, either as hollow outgrowths containing prolongations of the enteric cavity, which become the coelom, or as solid proliferations. But in some groups the mesoderm is actually laid down in cleavage, and is present at the end of that process. ”

“ Reproductive Organs.—In the cockroach embryo, before the segmentation of the germ-band has begun, the primitive germ-cells can be recognized at the hinder end of the mesoderm, from whose ordinary cells they can be distinguished by their larger size. At a later stage further germ-cells arise from the epithelium of the coelomic pouches from the second to the seventh abdominal segments, and become surrounded by other mesoderm cells which form the ovarian or testicular tubes and ducts (fig. 19, g) . ”

“ Cases of heterotopy, or of organs being developed in a position or a germ-layer other than that in which they originally arose in phylogeny, are not so easy to find. According to Haeckel, the origin of the generative products in the mesoderm is a heterotopic phenomenon, for he considers that they must have originated phylogenetically in one of the two primary layers, ectoderm or endoderm. ”

“ Thus certain embryologists supposed that this layer corresponded to the reproductive organs of primitive animals: others regarded it as the prototype of the organs of locomotion. My embryological and physiological studies on sponges led me to the conclusion that the mesoderm must function in the hypothetically primitive animals as a mass of digestive cells, in all points similar to those of the endoderm. ”

“ Great importance was therefore attached to them by embryologists, and both their mode of development and the part which they play in forming the adult organs were examined with the greatest care. It is very unusual for all the layers to be established at the same time. As a general rule the ectoderm and endoderm, which may be called the primary layers, come first, and later the mesoderm is developed from one or other of them. ”

“ Moreover, in their psychical development, certain insects are equal, or even superior, to any other animals, except birds and mammals.
The animal kingdom is primarily divided into two grand divisions, the one-celled (Protozoa) and many-celled animals (Metazoa) . In the latter group the cells and tissues forming the body are arranged in three fundamental cell-layers; viz. the ectoderm or outer layer, the mesoderm, and endoderm. ”

“ Germ-cells can thus only arise from the cells of the germ-track, and in no other way, no matter whether the germ-track lie in the ectoderm, as in the Hydromedusæ, or in the endoderm, as in true jellyfishes (Acalephæ) and the Ctenophores, or in the mesoderm, as in many higher groups of animals. ”

“ But very early this tissue is increased to form a massive layer by direct development from the mesothelium. The new mesodermal cells thus produced constitute the mesenchyma, which includes the whole of the mesoderm of the embryo except the mesothelial lining of the cœlom. The cells of the mesenchyma, connected with each other and with the mesothelial cells by protoplasmic processes, are not as close together as in an epithelium and do not form a continuous membrane. ”

“ While dealing with our harmonious-equipotential systems as the starting-points of processes of restitution, e.g. in Tubularia, Clavellina, the flatworms, and other instances, we always have regarded cross-sections of the body as constituting the elements of equipotentiality. Now cross-sections, of course, are by no means simple in themselves, but are made up of very different tissues, which are derivates of all three of the original germ layers—ectoderm, mesoderm, and endoderm. ”

“ As has been implied in the above account, mesenchyme is usually formed from epithelial mesoderm or from endoderm, or from tissue destined to form endoderm. It is also sometimes formed from ectoderm, as in the Vertebrata at the nerve crest and other places. In some Coelenterata also it appears certain that the ectoderm does furnish tissue of a mesenchymatous nature which passes into the jelly, but this phenomenon takes place comparatively late in life, at any rate after the embryonic period. ”

“ Figure 11i, four sections farther from the head, shows the same greatly thickened ectoderm (ec) with the same break (ec′) in the middle line. The section is posterior to the notochord and passes through the anterior edge of the blastopore or, as it may now perhaps better be called, the neurenteric canal. The cells of the medullary wall are continuous with those of the entoderm. The mesoderm (mes) is still distinct from the other germ layers. ”

“ Reproduction.—As regards the modes of reproduction, both male and female cells are found in the mesoderm. The male cells generally give rise by division of the nucleus to masses of spermatozoa, each of which possesses a conical head and a long vibratile filament. The ova appear as large round cells, and when conglomerated in masses, resemble those of Micro-gromia, which, after fertilisation, undergo segmentation or division, first into two cells, and again dividing and sub-dividing, until a cluster or mass of cells results (as 511 seen in Fig. 343) . ”

“ He finds that the endoderm arises from an anterior and a posterior rudiment derived from the “endoblast,” that many of the cells of these rudiments wander into the yolk, and that the mesenteric epithelium becomes reinforced by cells that migrate from the yolk. K. Escherich (1901) , after a new research on the embryology of the muscid Diptera, claims that the fore and hind endodermal rudiments arise from the blastoderm by invagination, and are from their origin distinct from the mesoderm. ”

“ The embryo is much more compressed, in a dorso-ventral direction, and the medullary groove (mg) is correspondingly more shallow. Where the ectoderm (ec) curves over to form the medullary folds it becomes much more compact and somewhat thicker. The notochord (nt) is large and distinct, but is still fused with the entoderm (en) . The mesoderm (mes) forms a well-defined layer, entirely distinct from both the notochord and the entoderm. From this region, as we pass caudad, the size of the embryo in cross-section gradually decreases and the medullary groove becomes more shallow. ”

“ The glandular lining of the alimentary canal corresponds with the endoderm of Hydra; although this, too, is a more complicated affair. The mass of the body, lying between these two layers, is considered to correspond somewhat with the mesoderm of Grade III., and has received the collective term of Mesoblast. This description applies equally to the earthworm, for the higher worms differ immensely from the lower worms, and stand on a level with more important members of the animal kingdom (see Fig. 41, p. 139) . The body-cavity may be formed in different ways in different animal groups ”

“ I consider it an especial merit of the theory that it has established the identity of the development of the two middle layers in all the vertebrates, and has traced them as cenogenetic modifications back to the original palingenetic form of development that we still find in the amphioxus. Carl Rabl comes to the same conclusion in his able Theory of the Mesoderm, and so do Ray-Lankester, Rauber, Kupffer, Ruckert, Selenka, Hatschek, and others. ”

“ We have now arrived at the point (phylum Echinodermata, "spiny-skinned") where a distinctly new type of interior structure appears in the possession by animals of a hollow space (cœlom) between the outer skin and the wall of the digestive tube which now becomes occupied by definite organs instead of by an almost uniform mesenchyme, as in the sponges and cœlenterates. These organs arise from an interior lining membrane called "mesoderm." ”

“ These cells draw in their flagellum, become amœboid and mobile, multiply by division, fill the cavity of the blastula, and become capable of digesting. They originate the digestive cells of the complete organism and give birth to the mesoderm, which explains how the latter comes to contain a number of devouring cells even though these do not constitute digestive organs properly so called. Metchnikoff gave to that stage the name of parenchymella, for the migrating cells constitute the endoderm in the condition of a parenchyma. ”

“ Embryology.—As the mesoderm is gradually spreading over the embryo it splits into two layers, the outer of which is known as the somatopleure and lines the parietal or ectodermal wall, while the inner lines the entoderm and is called the splanchnopleure ”

“ The outer grey part of the body, which is attached at or near its The Test. posterior end and penetrated by the two apertures, is the “test.” This is a firm gelatinous cuticular secretion upon the outer surface of the ectoderm, which is a layer of flat cells. Although at first produced as a cuticle, the test soon becomes organized by the migration into it of cells derived from the mesoderm. A. Kowalevsky has shown that cells of the mesenchyme of the larva make their way through the ectoderm to the exterior during the metamorphosis, and become the first cells of the young test. ”

“ This hypothesis necessarily attracted my attention to the power of seizing foreign corpuscles possessed by the mesodermic cells” (Immunity in Infective Diseases, English translation, Cambridge, 1905) .
The branch of embryology which concerns itself with the study of the origin, history and conjugation of the individuals (gametes) which are concerned in the reproduction of the species has made great advances. These began in 1875 and following years with a careful examination of the behaviour of the germinal vesicle in the maturation and fertilization of the ovum. ”