The elements of Embryology, (1874) The elements of Embryology, elementsofembryo74fost Year: 1874 VII.] THE INVESTING MASS. 177 3. As we mentioned in the last chapter, the formation of the primitive cranium commenced upon the fourth day. This in its earliest stage, inasmuch as it is composed of con- densed but otherwise only slightly differentiated mesoblast, may be spoken of as the membranous cranium. On the sixth day, true hyaline cartilage makes its ap- pearance ; and the primitive membranous cranium gives place to the primitive cartilaginous cranium. The cartilage which is the first to app
The elements of Embryology, (1874) The elements of Embryology, elementsofembryo74fost Year: 1874 VII.] THE INVESTING MASS. 177 3. As we mentioned in the last chapter, the formation of the primitive cranium commenced upon the fourth day. This in its earliest stage, inasmuch as it is composed of con- densed but otherwise only slightly differentiated mesoblast, may be spoken of as the membranous cranium. On the sixth day, true hyaline cartilage makes its ap- pearance ; and the primitive membranous cranium gives place to the primitive cartilaginous cranium. The cartilage which is the first to appear, forms a thick plate called the investing mass of Rathke (Fig. 55, iv.), sur- rounding the whole of that portion of the notochord which projects in front of the foremost pi-otovertebra. The hinder Pig. 55. View from above or the Investing Mass and of the Trabecule on THE Fourth Day of Incubation. (From Parker.) In order to shew this, the whole of the upper portion of the head has been sliced away. The cartilaginous portions of the skull are marked with the dark horizontal shading. cv. I. cerebral vesicles (sliced off), e. eye. nc. notochord. iv. investing mass. 9. foramen for the exit of the ninth nerve, cl. cochlea, hsc. horizontal semicircular canal, q. quadrate. 5. notch for the passage of the fifth nerve. Ig. expanded anterior end of the investing mass. pts. pituit try space. tr. trabeculae. The reference line tr. has been accidentally made to end a little short of the cartilage. E. 12
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