78 THE EARI.Y DEVELOPMENT OF MAMMALS. 



nated, producing a cup, the wall of which is necessarily double. The layer next the 

 cavity of the cup increases in thickness and becomes the retina proper. The other 

 layer remains thin and becomes charged with pigment, and is transformed into the 

 so-called pigment layer of the retina. The opening of the cup is filled with the 

 lens (compare also Fig. 17). In accordance with the fact that the optic nerve 

 and retina are derived from the wall of the medullary tube r we find that their dif- 

 ferentiation is essentially similar to that of the central nervous system. They 

 develop neuroglia and nerve-cells with neuraxons. It is only by keeping in mind 

 these facts that the histogenesis of the adult structure of the retina can be 

 understood. 



Lens. The visual fllakode, which, it will be remembered, is at first in contact 

 with the wall of the optic vesicle, is early invaginated (Fig. 153, L). The invagi- 

 nation closes over, making the lentic vesicle; this rapidly separates from the over- 

 lying ectoderm, which ultimately becomes the corneal epithelium. The wall of the 

 vesicle which is nearest the retina and farthest from the epidermis rapidly thickens 

 and forms the main substance of the lens, at the same time obliterating the cavity 

 of the vesicle. 



Mesoderm. The mesoderm produces the choroid and sclerotic coats of the 

 adult eyeball, the connective tissue of the iris and of the cornea, the lining epi- 

 thelium of the anterior chamber of the eye, and the muscles which move the 

 eye-ball. 



For further details as to the history of the eye see page 331. 



The Otocyst. 



The otocyst arises by the invagination of the auditory plakode (Fig. 41, Plk) 

 to form the auditory pit "(Fig. 152, Of), which soon closes, forming an epithelial 

 vesicle which quickly loses its connection with the overlying ectoderm. The vesicle 

 or otocyst is the anlage of the membranous labyrinth of the ear. It lies close to 

 the wall of the myelencephalon (Fig. 191, Of) between the ninth nerve and the 

 ganglion complex of the seventh-eighth nerves. It becomes pear-shaped, the nar- 

 row end pointing dorsally. It soon develops a special prolongation (Fig. 42, 

 D.endo) which extends dorsad near the brain-wall and is known as the ductus 

 endolymphaticus. The ductus persists throughout life. 



The ventral end (Cock) of the otocyst begins to elongate very early, and is 

 converted gradually into a very long spiral epithelial tube, the scala media cochlea 

 of the adult. The 'dorsal summit, S.C, of the otocyst is transformed into the 

 semicircular canals. The middle part of the original vesicle also undergoes remark- 

 able changes of form to produce the utriculus, which opens into the anterior and 

 external semicircular canals; the canalis reuniens, which leads to the cochlea; and 

 the saccidus, a blind pouch on the anterior side of the canalis reuniens. 



The entire epithelial labyrinth becomes surrounded by a loose mesenchyma, 

 which again is surrounded by denser tissue which forms the cartilaginous periotic 



