. The development of the chick; an introduction to embryology . owth until they meet over thekeel and fuse above it in a mass in which the outer and innerlayers of the retina are indistinguishably fused. Thus the proxi-mal portion of the mesodermal keel is enclosed in a kind of tunnelcomposed of the involuted edges of the optic cup. The forma-tion of this tunnel {progresses gradually from the fundus towards 282 THE DEVELOPMENT OF THE CHICK the ora serrata by the same process of invohition, until on theeighth (lay the mesodermal keel is completely covered up. Fig. 162 gives a diagrammatic view
. The development of the chick; an introduction to embryology . owth until they meet over thekeel and fuse above it in a mass in which the outer and innerlayers of the retina are indistinguishably fused. Thus the proxi-mal portion of the mesodermal keel is enclosed in a kind of tunnelcomposed of the involuted edges of the optic cup. The forma-tion of this tunnel {progresses gradually from the fundus towards 282 THE DEVELOPMENT OF THE CHICK the ora serrata by the same process of invohition, until on theeighth (lay the mesodermal keel is completely covered up. Fig. 162 gives a diagrammatic view of the condition of thepecten in the middle of the seventh day of incubation. and 164 show sections through this at the points a, b, c, d, e,indicated in the figure. The formation of the tunnel will bereadily understood by study of the figures. It will be seen thatthe major portion of the embryonic pecten is of ectodermal origin,and that the mesoderm forms a relatively inconspicuous partof it. Later, on the same day, it becomes increasingly difficult. Fig. 162. — l)i;i<:r:iinmatic reconstruction of the pecten of theeye of a chick embryo of 1\ ilays incubation. (After Bernd.) Ch. fis. 1., Lip of the choroid fissure. Ch. fiss., Choroid fis-sure. Mes., Mesobhist. Mes. b., Boundary of the mesoblastwithin the choroid fissure. Mes. K., Thickening of the meso-blastic keel. op. C, Optic cup. O. St., Optic stalk. P., Pec-ten. P. B., of the pecten. The arrow indicates the direction of growth of the ecto-dermal tunnel. The lines a, b, c, d, e show the planes of the sections re-produced in Fig. 163 (a, b, c, c) and in Fig. 164 (d). to distinguish ectodermal and mesodermal portions of the pecten,and thereafter it is quite impossible to sa\- which ])arts of it areof ectodennal and which are of mesodermal origin. During theeighth and ninth days the i)ecten increases greatly in height,and hcconics relatively very much narrower. The folds of tlie pecten now begin to develo])
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