282 THE DEVELOPMENT OF THE CHICK 



the ora serrata by the same process of involution, until on the 

 eighth day the mesodermal keel is completely covered up. 



Fig. 162 gives a diagrammatic view of the condition of the 

 pecten in the middle of the seventh day of incubation. Figs. 

 163 and 164 show sections through this at the points a, b, c, d, e, 

 indicated in the figure. The formation of the tunnel will be 

 readily understood by study of the figures. It will be seen that 

 the major portion of the embryonic pecten is of ectodermal origin, 

 and that the mesoderm forms a relatively inconspicuous part 

 of it. Later, on the same day, it becomes increasingly difficult 



FIG. 162. Diagrammatic reconstruction of the pecten of the 

 eye of a chick embryo of 7 days' incubation. (After Bernd.) 



Ch. fis. 1., Lip of the choroid fissure. Ch. fiss., Choroid fis- 

 sure. Mes., Mesoblast. Mes. b., Boundary of the mesoblast 

 within the choroid fissure. Mes. K., Thickening of the meso- 

 blastic keel. op. C., Optic cup. O. St., Optic stalk. P., Pec- 

 ten. P. B., Base 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, e) and in Fig. 164 (d). 



to distinguish ectodermal and mesodermal portions of the pecten, 

 and thereafter it is quite impossible to say which parts of it are 

 of ectodermal and which are of mesodermal origin. During the 

 eighth and ninth days the pecten increases greatly in height, 

 and becomes relatively very much narrower. 



The folds of the pecten now begin to develop and, by the 

 seventeenth day their number is 17-18, the same as in the adult. 

 The pigment does not begin to appear until about the twelfth day. 

 The details of the development of the blood-vessels are not known. 



