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Fig. 460. Section through head of chick of two days' incubation


The formation of the optic vesicle and stalk appears to be somewhat more advanced on the left than on the right.

The first indication of eye formation is found in the chick at the beginning of the second day of incubation ; in the human embryo, at what has been estimated as about the second or third week. At this stage the neural canal is not yet completely closed in and its anterior end shows three primary brain vesicles (p. 440, Fig. 497). The anlagen of the eyes first appear as bilaterally symmetrical evaginations from the lateral walls of the fore-brain vesicle (Figs. 459 and 460), and are at first large in proportion to the brain vesicle itself. When first formed, the optic evagination opens widely into the fore-brain vesicle (Fig. 460, right side), but as the distal part of the evagination expands more rapidly than the proximal part, there soon results a spheroidal optic vesicle attached to the fore-brain by the narrow optic stalk (Fig. 460, left side) . Through the latter the cavity of the optic vesicle and the cavity of the fore-brain are in communication. With the development of the hemispheres, that part of the brain to which the optic stalks are attached becomes the inter-brain (diencephalon).

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Bailey FR. and Miller AM. Text-Book of Embryology (1921) New York: William Wood and Co.

Cite this page: Hill, M.A. (2024, April 22) Embryology Bailey460.jpg. Retrieved from

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