THE EMBRYO. 



1183 



epiblast in the groove between the lateral nasal and maxillary processes. This 

 thickening becomes hollowed out into a channel, and the lips of the groove meet 



Rudiment of choroid. 

 Rectus muscle, 



Oplic nerve. 



Retina. 



Pigment layer. 



Vitreous body 



(shrunken). 



Cavity of hind brain. 



-Eyelid. 



- -Lens. 



"" Cornea. 



- Membrana 

 pupillaris. 



Eyelid. 



'Iris. 



Pars ciliaris and pars iridica retinie. 

 FIG. 734. Horizontal section through the eye of an eighteen days' embryo rabbit ; x 30. (KOlliker.) 



over it, enclose it, and convert it into a duct, which eventually opens into the 

 nasal fossa. 



Development of the Ear. The first rudiment of the ear appears shortly after 

 that of the eye, in the form of a thickening of the epiblast, on the outside of that 

 part of the third primary cerebral vesicle which 

 eventually forms the medulla oblongata. The 

 thickening is then followed by an involution of 

 the epiblast (Fig. 735), which becomes deeper 

 and deeper, and sinking toward the base of the 

 skull, forms a flask-shaped cavity ; by the nar- 

 rowing of the external aperture the neck of 

 the flask constitutes the recessus labyrinthi. 

 The mouth of the flask then becomes closed, 

 and thus a shut sac is formed, the primitive 

 auditory or otic vesicle (Fig. 736), which by its 

 sinking inward comes to be placed between the 

 ali-sphenoid and basi-occipital matrices. From it 

 the epithelial lining of the labyrinth is formed. 

 The primary otic vesicle becomes imbedded in a mass of mesoblastic tissue, 

 which rapidly undergoes chondrification and ossification. The vesicle is at first 

 flask- or pear-shaped ; the neck of the flask, or recessus labyrinthi, prolonged 

 backward, forms the aqugeductus vestibuli. From it are given off certain pro- 

 longations or diverticula, from which the various parts of the labyrinth are formed. 

 One from the anterior end gradually elongates, and, forming a tube, bends on 

 itself and becomes the cochlea. Three others, which appear on the surface of the 

 vesicle, form the semicircular canals, of which the external canal is the last to be 

 developed (Figs. 738 and 739). Subsequently a constriction takes place in the 

 original vesicle, which nearly divides it into two, and from these are formed the 

 utricle and saccule (Fig. 739). Finally, the auditory nerve, which has been devel- 



Auditory pit. 



Epiblast. 



Notochord. 



FIG. 735. Section through the head of 

 a human embryo, about twelve days old, 

 in the region of the hind-brain. (Koll- 

 mann.) 



