. The development of the chick; an introduction to embryology . ellings of the side-walls of the neural folds before the true brainvesicles are indicated, and last only about thn>e hours in the chick(twenty-fourth to twenty-seventh hours of iiicul»ation). Their exist-ence supports the liyi)()tliesis that the vertel^rate eyes are segmental, andthat the ancestors of v(ntebrates were primitivel)^multiple-e3ed. (Locy.) The external surface of the oi)tic vesicle early leaches theectoderm, to which it appears to l)e cemented at the 10 s the 17-18 s stage, tiie o])tic vesicles ]ir()ject d
. The development of the chick; an introduction to embryology . ellings of the side-walls of the neural folds before the true brainvesicles are indicated, and last only about thn>e hours in the chick(twenty-fourth to twenty-seventh hours of iiicul»ation). Their exist-ence supports the liyi)()tliesis that the vertel^rate eyes are segmental, andthat the ancestors of v(ntebrates were primitivel)^multiple-e3ed. (Locy.) The external surface of the oi)tic vesicle early leaches theectoderm, to which it appears to l)e cemented at the 10 s the 17-18 s stage, tiie o])tic vesicles ]ir()ject decidedly lieliindthe attachment of the optic stalk, and the external wall is slightlythicker than that uoxt the brain. Tlie ectoderm then becomesthickened over a circular area in contact with the optic vesicle FROM TWEL\E TO THIRTY-SIX SOMITES 165 and this constitutes the priniordium of the lens (Fig 94). Thethickening of the external wall of the optic vesicle and of thelens primordium now proceed rapidly, and soon an invaginationis formed in each (Fig. 95).. Fig. 94. — Section throughthe primordium of the eyeof a chick embryo of 21 s.(After FrorJep.) d., Distal wall of opticvesicle, p., Proximal wallof optic vesicle. Fic;. 95. - Section through theprimortlium of the eye of achick embryo at the end ofthe second day of incubation.(After Froriep.) It is probable that a stimulus is exerted by the optic vesicle on theectoderm with which it is in contact, causing it to thicken and becomethe primordium of the lens. This has been demonstrated experimentallyto be the case in the embryo of the frog, and the morphological rela-tions are the same in the chick. The invagination of the primarj opticvesicle to form the secondary optic vesicle is not mechanically producedby the growth of the lens, as some have supposed, for it has been shown(see Fol and Waryiisky) that the secondary ojjti(; vesicle is formed inthe absence of the lens. We may now consider the formation of the optic cup antl
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