. Cunningham's Text-book of anatomy. Anatomy. Xeuxal tube Ectoderm of amnion Mesoderm amnion Trophoblast of chorion Paraxial mesoderm Jntermediate cell tract Splanchnic and /somatic layers of lateral rjlate meso. Embryonic coelom Amnion Xeural crest Caudal neuropore. Xotochord Mesoderm of yolk-sac / Entoderm of yolk-sac -Amnion cavity N>Extra-em- bryonic ccelom Alimentary canal Yolk-sac Fig. 43. A. Diagram of a transverse section of a zygote, in which the neural tube has formed but has not separated from the surface ectoderm. B. Diagram of embryonic area of same zygote. Compare with surface
. Cunningham's Text-book of anatomy. Anatomy. Xeuxal tube Ectoderm of amnion Mesoderm amnion Trophoblast of chorion Paraxial mesoderm Jntermediate cell tract Splanchnic and /somatic layers of lateral rjlate meso. Embryonic coelom Amnion Xeural crest Caudal neuropore. Xotochord Mesoderm of yolk-sac / Entoderm of yolk-sac -Amnion cavity N>Extra-em- bryonic ccelom Alimentary canal Yolk-sac Fig. 43. A. Diagram of a transverse section of a zygote, in which the neural tube has formed but has not separated from the surface ectoderm. B. Diagram of embryonic area of same zygote. Compare with surface view of embryo in Fig. 38. The Cephalic Mesoderm.—It has already been noted that the mesoderm of the head becomes segmented only in the region of the caudal part of the hind-brain, where four cephalic mesodermal somites are formed on each side. From the scleratogenous portions of these somites are developed the occipital part of the skull and the corresponding portions of the membranes of the brain, and from their muscle plates the intrinsic muscles of the tongue. The unsegmented part of the cephalic mesoderm gives rise to the remaining muscles and connective tissues of the head region. Early Stages of the Development of the Nervous System.—No definite trace of the nervous system is present until the primitive streak has formed and the embryonic area has passed from a circular to an elongated form. Then an area of thickened ectoderm, the neural plate, appears in the anterior part of the area. It commences a short distance posterior to the anterior end of the area, and its posterior extremity embraces the anterior end of the primitive streak. Its lateral margins fade gradually into the surrounding ectoderm, and in the earliest stages cannot be definitely defined; but, as the elongation of the plate continues coinci- dently with the elongation of the embryonic area, the lateral margins of the plate are elevated as the mesoderm beneath them thickens, and so they become distin
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectanatomy, bookyear1914
