. The development of the frog's egg; an introduction to experimental embryology. Embryology. Ch. XV] ORGANS FROM THE ECTODERM 163 leaves the capsule. These auditory vesicles separate from the surface ectoderm. "At the time of the separation the vesicle is a closed sac somewhat pyriform in shape; its lower or ventral portion being spheri- cal and lying opposite the notochord, and its dorsal wall l)eing prolonged up- wards into a short blind diverticulum lying at the side of the hind-brain. The wall of the vesicle consists of a single layer of cubical ' "^"^^ ,>, or colunniar c
. The development of the frog's egg; an introduction to experimental embryology. Embryology. Ch. XV] ORGANS FROM THE ECTODERM 163 leaves the capsule. These auditory vesicles separate from the surface ectoderm. "At the time of the separation the vesicle is a closed sac somewhat pyriform in shape; its lower or ventral portion being spheri- cal and lying opposite the notochord, and its dorsal wall l)eing prolonged up- wards into a short blind diverticulum lying at the side of the hind-brain. The wall of the vesicle consists of a single layer of cubical ' "^"^^ ,>, or colunniar ; This ectodermal sac becomes the Fig. so. — Cross-section through hind- T . r ii • brain (H) and inner ear (E). N. sensory lining of the inner Notochord. ear (Fig. 50). The Nerves At the time when the medullary plate forms as a thickening of the ectoderm, there also forms, as we have seen, on each side of the plate a lateral neural ridge or jylate of ectoderm. Each neural ridge appears at first as a continuation of one side of the thickened medullary plate (Fig. 26). A slight constric- tion on each side marks the line of demarcation between the medullary plate and the neural ridge (Fig. 42). The neural ridges are more conspicuous at the anterior end of the medul- lary plate; they also develop somewhat earlier in this region. After the medullary plate has rolled up to form the medullary tube, the lateral neural ridges are also carried up, retaining for a time their primitive connection with the outer (now dorsal) part of the medullary tube (Fig. 40). The neural ridges next become broken up into a series of dorsal nerves, the cells collecting at certain regions, and thin- ning out and disappearing in the intermediate regions. The dorsal nerves grow down later between the myotomes and the nerve-cord. Accumulations of cells occur at certain regions on each dorsal nerve to form the ganglion of the dorsal root, and nerve-fibres are spun out from the cells of the gangli
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectembryology, bookyear1
