THE LATER DEVELOPMENT OF THE FROG 149 



stroma of the vitreous humor or body. In its essential com- 

 position it thus resembles a typical cranial nerve ganglion which 

 is derived from placode cells and neural crest; of course, in this 

 region, the neural crest is not typically present, but the retinal 

 cells have somewhat similar relations to the neural tube proper. 

 A day or two before hatching the choroid fissure of the optic 

 cup begins to close, first in the fundus region. Blood vessels 

 have already entered the posterior chamber and the margins of 

 the fissure embrace these. The region last to fuse, at the 

 margin of the cup, becomes enlarged as the ventral choroid 

 knot, and from this the iris develops, gradually extending 

 dorsally around the outer side of the pupil. The cornea and 

 outer coats of the eye-ball, as well as muscles are formed 

 from the mesenchyme around the optic cup. 



2. The Ear 



Like the eye this organ is a complex, derived from diverse 

 sources; the essential parts, composing the membranous laby- 

 rinth, are derived from the ectoderm of the surface of the head, 

 while the accessory parts, tubo-tympanic cavity and columella, 

 are of endodermal and mesodermal origin. We shall describe 

 first the formation of the membranous labyrinth or internal 

 ear. 



We have already mentioned the first formation of the auditory 

 organ as the auditory placode, which appears just as the neural 

 folds close together (Fig. 45, C). As the embryo begins to 

 elongate each of these placodes becomes depressed below the 

 surface of the head and invaginates, forming the ovoid auditory 

 sac or otocyst, about 0.2 mm. in diameter (Fig. 51, A). For 

 a time this remains connected with the surface of the head by a 

 narrow tube of cells, but just before hatching it becomes com- 

 pletely closed and separates entirely from the surface ectoderm, 

 sinking in toward the lateral surface of the myelencephalon 

 (Fig. 51, B). The superficial layer of the ectoderm does not 

 share in the formation of the auditory sac but remains con- 

 tinued across the surface of the head outside the invaginating 



