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Next there arise within it a number of little bodies crystalline in form, and which actually are wax or oil crystals. They increase with great rapidity, the inner sac or mesoblast becoming sometimes so crowded with them, that its shape is affected by the protrusion of their angles.
But in 1891 Pawkins, whose health had been bad for some time, published some work upon the "mesoblast" of the Death's Head Moth. What the mesoblast of the Death's Head Moth may be does not matter a rap in this story. But the work was far below his usual standard, and gave Hapley an opening he had coveted for years. He must have worked night and day to make the most of his advantage.
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.
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.
But in 1891 Pawkins, whose health had been bad for some time, published some work upon the "mesoblast" of the Death's Head Moth. What the mesoblast of the Death's Head Moth may be, does not matter a rap in this story. But the work was far below his usual standard, and gave Hapley an opening he had coveted for years. He must have worked night and day to make the most of his advantage.
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.
And now we have a perfect cell-structure, differing from an ordinary cell only in having the inner sac, inclosing the dot, on the side, instead of in the centre. The outer membrane corresponds to the Ectoblast, or outer cell sac, the Purkinjean vesicle to the Mesoblast, or inner cell sac, while the dot in the centre answers to the Entoblast.
In the further course of development the epiblast becomes the ectoderm or epidermic layer of the body; the hypoblast becomes the epithelium of the middle portion of the alimentary canal; and the mesoblast gives rise to all the other tissues, except the central nervous system, which originates from an ingrowth of the epiblast.
In the language of our science, the outer envelope is called the Ectoblast, the inner sac the Mesoblast, and the dot the 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. When the mesoblast has become thus infinitely subdivided into hundreds of minute spheres, the ectoblast bursts, and the new generations of cells thus set free collect in that part of the egg where the embryonic disk is to arise.
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