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Fig. 30. Profile views of the brains of human embryos at third, fourth, and eighth weeks

Profile as seen during the third (A), fourth (B), and eighth (C) weeks, showing the conversion of the three primary cerebral vesicles into their chief subdivisions and the formation of the flexures of the neural tube. (After Wilhelm His (1831-1904))

A, optic vesicle; Br, pontine region; Gb, auditory vesicle; H, telencephalon; Hb, metencephalon; J, isthmus; M, mesencephalon; N, myelencephalon; NK, neck bend; Pm, mammillary recess; Rf, posterior medullary velum; Tr, infundibular recess; Z, diencephalon.

The formation of the flexures of the neural tube is shown in Fig. 30. It will be seen that there are three distinct flexures, cephalic, pontine and cervical. Two of them have already been mentioned. The third, or cervical flexure, marks the junction of brain and spinal cord and is formed about the same time as the pontine flexure. They are formed, in part at least, in consequence of unequal growth of different parts of the neural tube. They probably influence and also are influenced by the growth of the surrounding structures. The cephalic and cervical flexures involve the surrounding structures to a considerable extent so that there is a corresponding bend of the axis of the whole head, and thus the presence of them can be recognized on the exterior of the embryo. The pontine flexure, however, is limited to the nervous system. The cephalic flexure persists into adult life. The pontine flexure finally disappears and the cervical flexure nearly does.

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Streeter GL. The Development of the Nervous System. (1912) chapter 14, vol. 2, in Keibel F. and Mall FP. Manual of Human Embryology II. (1912) J. B. Lippincott Company, Philadelphia.

XIV. The Development of the Nervous System: The Histogenesis of Nervous Tissue | The Development of the Central Nervous System | The Peripheral Nervous System | The Sympathetic Nervous System | Manual of Human Embryology II
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Cite this page: Hill, M.A. (2018, August 19) Embryology Keibel Mall 2 030.jpg. Retrieved from

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