Archive image from page 184 of The development of the chick;. The development of the chick; an introduction to embryology . developmentofchi00lill Year: 1908 FROM TWELVE TO THIRTY-SIX SOMITES 165 and this constitutes the primordium of the lens (Fig 94). The thickening of the external wall of the optic vesicle and of the lens primordium now proceed rapidly, and soon an invagination is formed in each (Fig. 95). Fig. 94. — Section through the primordium of the eye of a chick embryo of 21 s. (After Froriep.) d., Distal wall of optic vesicle, p., Proximal wall of optic vesicle. Fig. 95. — Secti
Archive image from page 184 of The development of the chick;. The development of the chick; an introduction to embryology . developmentofchi00lill Year: 1908 FROM TWELVE TO THIRTY-SIX SOMITES 165 and this constitutes the primordium of the lens (Fig 94). The thickening of the external wall of the optic vesicle and of the lens primordium now proceed rapidly, and soon an invagination is formed in each (Fig. 95). Fig. 94. — Section through the primordium of the eye of a chick embryo of 21 s. (After Froriep.) d., Distal wall of optic vesicle, p., Proximal wall of optic vesicle. Fig. 95. — Section through the primordium of the eye of a chick embryo at the end of the second day of incubation. (After Froriep.) It is probable that a stimulus is exerted by the optic vesicle on the ectoderm with which it is in contact, causing it to thicken and become the primordium of the lens. This has been demonstrated experimentally to be the case in the embryo of the frog, and the morphological rela- tions are the same in the chick. The invagination of the primary optic vesicle to form the secondary optic vesicle is not mechanically produced by the growth of the lens, as some have supposed, for it has been shown (see Fol and Warynsky) that the secondary optic vesicle is formed in the absence of the lens. We may now consider the formation of the optic cup and of the lens separately. The Optic Cup. The invagination of the outer wall of the primary optic vesicle gradually brings this wall into contact with the inner wall and obliterates the primary cavity. Thus
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