. Text-book of embryology. Embryology. 110 EMBEYOLOGY OF THE LOWEE VEETEBEATES ch. n.~ llJ Xt. recede from the spinal cord, but yet each motor nerve is already present as a distinctly-fibrillated trunk bridging across the narrow gap between spinal cord and myotome. A few mesenchyme cells have wandered into the gap but they have not yet begun to con- centrate round the nerve-trunk. Fig. 60, A is taken from an embryo of stage 24 at a time when myotome and spinal cord are still in close contact with one another. In specimens which were extended in one plane under normal salt solution while still
. Text-book of embryology. Embryology. 110 EMBEYOLOGY OF THE LOWEE VEETEBEATES ch. n.~ llJ Xt. recede from the spinal cord, but yet each motor nerve is already present as a distinctly-fibrillated trunk bridging across the narrow gap between spinal cord and myotome. A few mesenchyme cells have wandered into the gap but they have not yet begun to con- centrate round the nerve-trunk. Fig. 60, A is taken from an embryo of stage 24 at a time when myotome and spinal cord are still in close contact with one another. In specimens which were extended in one plane under normal salt solution while still alive and subjected to the action of the fixing agent in that position, it is found that the myotome is frequently pulled slightly away from the spinal cord (as in the specimen figured) and in such cases it is found that the nerve- trunk already exists in the form of a bridge of soft granular protoplasm (n) with- out any trace of fibrillation, connecting spinal cord and myotome. That these bridges are really the nerve-trunks is indicated by their occur- rence one to each myotome, apart from the fact that a con- tinuous series of stages have been observed between them and the fully developed nerve-trunks. In summing up we may take the various stages in their proper chronological sequence. (1) The nerve-trunk is already present as a proto- plasmic bridge at a period so early in development that spinal cord and myotome are still in contact with one another. (2) As the embryo grows and the myotome recedes from the spinal cord this protoplasmic bridge increases in length and becomes fibrillated. (3) As the nerve-trunk lengthens amoeboid masses of mesen- chymatous protoplasm collect round it and gradually spread out over its length to form the protoplasmic sheath. In stages later than those figured the sheath protoplasm insinu- ates itself in amongst the nerve-fibrils of the trunk, dividing them up into bundles or nerve-fibres. As the myotome resolves itself into the various muscles
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Keywords: ., bookcentury1900, bookdecade1910, bookpubli, booksubjectembryology
