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Fig. 74. Section through very young human chorionic vesicle embedded in the uterine mucosa


The vesicle measured 2.4 x 1.8 mm, the embryo 19 mm. Peters reckoned the age as 3 or 4 days, but later studies of other embryos go to show that the age is much greater. Bryce and Teacher estimate it at 14 to 15 days.

In a slightly older embryo described by Peters (Fig. 74) a space has appeared within the mesoderm, so that one portion remains as a lining for the trophodermal wall and the remainder closely invests the yolk sac and amnion and also forms a layer between ectoderm and entoderm in the embryonic disk (Fig. 75). The disk is therefore composed of all three germ layers, but there is still no indication of a primitive streak. It would seem that in the highest primate the mesoderm develops independently of the primitive streak; but whether it arises from ectoderm or entoderm it is not possible in the present state of our knowledge to determine.

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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 25) Embryology Bailey074.jpg. Retrieved from

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