In the vertebrate the segmentation in this region is indicated by the coelomic or head-cavities, which are cavities formed in the mesoderm of the embryo, the walls of which give origin to the striated muscles of the head.
Do the mesoderm segments, which give origin to the eye-muscles, and therefore do the head-cavities of this region, correspond with the trigeminal segments?
As to the segments of the mesoderm in the head, the three hindmost or occipital in Petromyzontidae remain permanently, and correspond to the three last segments in the selachian head.
Ahlborn, in 1884, drew a sharp distinction between the segments of the mesoderm and those of the endoderm.
Killian comes to much the same conclusion as Dohrn, for he finds seventeen to eighteen separate mesoderm segments in the head, of which twelve belong to the trigeminal and facial region.
Of the anterior mesoderm segments, he considered that there were originally six, and that there are six typical eye-muscles in all Craniota, which have been compressed into three segments, as in Selachia.
Afterwards they painted their armor a dark grey, like chunks of mesoderm stone.
They saw their first asteroid--a pitted, mesoderm fragment of nickel-iron from middle-deep in the blasted planet.
Twenty miles off was a haze against the stars--a cluster of small mesoderm fragments.
This is particularly clearly seen in the case of the anterior part of the head of Elasmobranch and probably of other vertebrate embryos, where all the mesoderm present is derived from the anterior part of the neural crest (Quart.
The difficulties in the application of the layer-theory become vastly greater when the origin and fate of the mesoderm is considered.
Sometimes the epithelial mesodermis the first formed, and what little mesenchyme there is is developed from it (Amphioxus, Balanoglossus, &c.
It is clear, both from their origin and history, that the layers or masses of cells called ectoderm, endoderm and mesoderm have not the same value in different animals; indeed, it is misleading to speak of three layers.
In Annelids the bulk of the mesoderm has at first a modified epithelial form similar to that of Arthropods, but it is formed, not from a primitive streak, but from some peculiar cells produced in cleavage, called pole-cells.
But in some groups the mesoderm is actually laid down in cleavage, and is present at the end of that process.
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.
It has been already stated that the mesoderm is a composite tissue.
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.
Sometimes the mesenchyme is the first to arise, the epithelial mesodermdeveloping from it (most, if not all, Vertebrates).
In the Vertebrata the whole of the mesodermhas at first the mesenchyme form.
As already hinted, in Balanoglossus and Amphioxus the whole of the mesoderm of the body is at first in an epithelial condition, being developed as an outgrowth of the gut-wall.
Some of them, like the choroidal on the mesial surface, are developed very early, while the vesicle is little more than epithelial, and contain between their walls an inpushing of mesoderm to form the choroid plexus.
Hence the mesoderm consists from the first of TWO genetically distinct layers, which do not originate by the cleavage of a primary simple middle layer (as Remak supposed).
The parts of the mesoderm at which the first traces of them are found are usually called the middle or mesenteric plates.
Now a third layer (mesoderm = middle skin) is formed, by the growth inwards of two pouches or folds of the skin.
These pairs of "cubes" of the mesoderm are the first traces of the primitive segments or somites, the so-called "protovertebrae.
As the first traces of the gonads make their appearance in the lining of these middle plates nearer inward (or the middle) from the inner funnels of the nephro-canals, it is better to count this part of the mesoderm with the hyposoma.
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.
It will be remembered that Remak introduced in 1855 the conception of the mesoderm as an independent layer derived from the endoderm.
The mesoderm is a plate of cells lying between the endodermic and ectodermic plates.
The mesoderm starts to segment to form the skin, and the neural canal (from which develop the spinal cord and brain) for the nervous system forms.
When the mesoderm develops into two plates, a cavity, called the Primitive Coelom, appears between the plates.
After the mesoderm is thus established, a little vesicle lying upon and open to the yolk is formed from the endoderm, and this vesicle ultimately gives rise to the stomach, the two lobes of the liver and the intestine.
The mesoderm also originates at the anal side of the ectoderm and extends in two bands right and left between ectoderm and endoderm.
The body cavity, which is a space between the ectoderm and alimentary canal, is not lined by mesoderm and is traversed by a few muscular fibres.
The pericardial area is early differentiated from the rest of the coelom and at first lies in front of the neural and bucco-pharyngeal area; here the mesoderm stretches right across the mid-line, which it does not in front and behind.
At the same time it is necessary to observe that it is by no means certain that the mesoderm found in various groups of Metazoa is a similar or homologous formation in all cases.
Into the dorsal mesentery the pancreas grows, also as diverticula, from the duodenum, while the spleen is developed from the mesoderm contained in the same fold.
In the higher vertebrates (Craniata) the coelom is developed by a splitting of the mesoderm into two layers, and a pericardium is constricted off from the general cavity.
Simultaneously with the construction of the medullary tube we have in the Amphioxus-embryo the formation of the chorda, the coelom-pouches, and the mesoderm proceeding from their wall.
In their more advanced mesoderm we find a few contractile longitudinal canals which force the blood through the body by their contractions; these are the first blood-vessels.
But between the two germinal layers a mesoderm is developed, a soft mass of connective tissue, in which the organs are embedded.
This vessel supplies a series of small branches to the intestine, which reach the same by passing ventrad imbedded in the mesoderm connecting the tube to the dorsal body wall.
Both the somatic and the splanchnic leaf of the mesoderm consist at first solely of a layer of flattened epithelial cells, the mesothelium.
As these coelom cavities develop in the mesoderm the cells lining them become distinctly epithelial.
The entodermal enteric tube is, as already stated, closely attached at an early period along its dorsal surface to the axial rod of mesoderm containing the chorda dorsalis immediately ventrad of the neural canal.
The entodermal tube and the surrounding splanchnic mesoderm forming the intestinal canal is attached along its dorsal surface to the axial mesoderm of the dorsal mid-line.
Early in the course of development a number of spaces appear in the mesodermon each side of the axial line of the embryo.
The connective tissue of the splanchnic mesoderm appears as an intermediate layer uniting the epithelial lining and the muscular walls.
Curves of blastodermic layers and division of mesoderm in amniote embryo.
The second chief function of the splanchnic and somatic mesoderm is the production of the serous membrane investing the body cavity and its contents from the mesothelium lining the primitive coelom.
The walls of this space are largely made up of the skeletal and muscular elements developed from the mesoderm of the somatopleure, covered superficially by the common ectodermal investment of the body.
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 coelom.
From the mesoderm arise the following structures:-- The connective tissues of the body.
We may confine our attention to this groove for the moment, leaving out of account the other two layers of the embryo--namely, the mesoderm and the entoderm.
Coelom-sac: the cavity containing the viscera: in embryology one of a pair of closed sacs, arising in the mesoderm of each segment of the embryo and giving rise to more or less of the coelom of the adult.
Endomesoderm: the inner layer formed by an invagination of the middle portion of the primitive band of the embryo, and from which the endoderm and mesoderm are subsequently differentiated.
He found that in certain inferior sponges themesoderm develops before the endoderm and gives birth to it.
The study of medusae and of their mesodermic digestion confirmed him more and more in the conviction that the mesoderm was a vestige of elements with a primitive digestive function.
The mass of the body, lying between these two layers, is considered to correspond somewhat with the mesoderm of Grade III.
On this view the entire food-canal in most Hexapoda must be regarded as of ectodermal origin, the "endoblast" represents mesoderm only, and the median furrow whence it arises can be no longer compared with the blastopore.
No doubt can be entertained that the greater part of the inner layer corresponds to the mesoderm of more ordinary embryos, for the coelomic pouches, the germ-cells, the musculature and the vascular system all arise from it.
Lepidosiren) walls of the mesoderm segment pass through a myoepithelial condition and give rise eventually to the great muscle segments (myomeres, or myotomes) which lie in series on each side of the trunk.
The ventral portion of the trunk mesoderm gives rise to the splanchnocoel or general coelom.
The development of the mesoderm of the head region is too obscure for treatment here.
The above list will hopefully give you a few useful examples demonstrating the appropriate usage of "mesoderm" in a variety of sentences. We hope that you will now be able to make sentences using this word.
