The mesenchyme in which they arise is segmental in origin (sclerotom), which suggests that they too may have been primitively segmental, but in existing fishes there are commonly two sets of arcualia to each body segment.
In the embryonic connective tissue or mesenchyme lying just outside the primary sheath of the notochord there are developed a dorsal and a ventral series of paired nodules of cartilage known as arcualia (fig.
In Scyphomedusae, in Ctenophora, and in Anthozoa, branched fixed and wandering cells are found in the mesogloea which seem to be the same thing as a good deal of what is distinguished as mesenchyme in Coelomocoela.
Thus Lankester, talking of mesenchyme, says: "I think we are bound to bring into consideration here the existence in many Coelentera of a tissue resembling the mesenchyme of Coelomocoela.
In this connexion it may be interesting to point out that in many Coelenterates all the tissues of the body retain throughout life the epithelial condition, nothing comparable to mesenchyme ever being formed.
In the Vertebrata the whole of the mesoderm has at first the mesenchyme form.
In Annelids with trochosphere larvae a certain amount of mesenchyme is formed at an earlier stage and gives rise to the muscular bands of the young larva.
Sometimes the mesenchyme is the first to arise, the epithelial mesoderm developing from it (most, if not all, Vertebrates).
Sometimes the epithelial mesoderm is the first formed, and what little mesenchyme there is is developed from it (Amphioxus, Balanoglossus, &c.
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.
Mesenchyme is the tissue which in Vertebrate embryology has frequently been called embryonic connective tissue.
In Echinodermata a certain amount of mesenchyme appears before the epithelial mesoderm, which is formed later as gut-diverticula.
The views of the Hertwigs depend to a large extent upon the supposition that it is possible to distinguish histologically muscle cells derived from epithelial cells, from those derived from mesenchyme cells.
In the Pseudocoela the muscular system has become differentiated from mesenchyme cells; while the body cavity, where it exists, is merely a split in the mesenchyme.
This mesenchymemay chondrify either in one piece (on each side) or in two; in Molge the part adjacent to the centrum is ossified in the 20-mm.
Figures of early stages in vertebral development by the authors mentioned show that the basidorsals chondrify first, as neural arches, while a separate mass of mesenchyme lies externally and ventrally from these.
Driesch[136] has pointed out that the mesenchyme cells are directed in their migration; and it seems that the direction of the growth of the axis cylinder is determined by the tissues into which it grows.
The young tadpoles have no legs, but the mesenchyme cells from which the legs are to grow out later are present at an early stage.
Driesch compared the size of the mesenchyme cells in a sea-urchin embryo produced by artificial parthenogenesis with those of a normally fertilized egg and found them half of the size of the latter.
No matter into which part of the body they are transplanted the mesenchyme cells for the foreleg will give rise to a foreleg only; even if they are transplanted into the spot from which the hindlegs grow out under natural conditions.
Thus in the tadpole earlymesenchyme cells are formed which are the anlagen for the four legs, which will grow out under the proper conditions.
These notches therefore contain cells comparable to seeds or to unfertilized eggs or to the mesenchyme cells which give rise to legs in the tadpole of the frog.
Not before the skeleton or mesenchyme is formed in the sea urchin egg is the influence of the nucleus noticeable.
Thus we see that the mesenchyme cells giving rise to legs may lie dormant for months or a year but will grow out when a certain type of substances, e.
Moreover, he could show that when two eggs were caused to fuse so as to produce a single larva of double size, the gastrulae of such larvae had twice the number of mesenchyme cells.
Certain outgrowths within the mesenchyme act as feeble muscles for lengthening and shortening the body and tentacles; but there are no blood vessels or excretory organs.
Now in animals superior to the jellyfishes and the flatworms, the mesenchyme is replaced by a definite hollow tissue that produces a more efficient system of muscular, excretory, and reproductive organs.
At the very same pole of the germ where the mesenchymecells originated there is a long and narrow tube of cell growing in, and this tube, getting longer and longer, after a few hours of growth touches the opposite pole of the larva.
The above list will hopefully give you a few useful examples demonstrating the appropriate usage of "mesenchyme" in a variety of sentences. We hope that you will now be able to make sentences using this word.
