124 DEVELOPMENT. 



(Fig. 94). At the point of involution the external layer of epiblast separates from 

 the ball of the lens and passes freely over the surface, so that the lens becomes dis- 

 connected from the epiblastic layer from which it was developed, and recedes into 

 the ocular cup, while the cuticular layer covering it is developed into the corneal 

 epithelium. The cells forming the posterior or inner wall of the cavity, whioh is to 

 form the lens, rapidly increase in size, becoming elongated and developed into 

 fibres, and, filling up the cavity, convert it into a solid body. The cells on the 

 anterior wall undergo no change and retain their cellular character. The secondary 

 ocular vesicle, or space between the lens and the hollow of the ocular cup (Fig. 94, 

 c 7, and 95), contains a quantity of mesoblastic tissue continuous through the ocular 

 cleft with the rest of the mesoblast, and into this blood-vessels project themselves 

 through the ocular cleft. The iris and ciliary processes are formed from this vas- 

 cular tissue, and the choroid is developed in the mesoblast surrounding the ocular 

 vesicle. A portion of this tissue also becomes converted into the vitreous humor, 

 and surrounds the lens with a vascular membrane the vascular capsule of the lens, 

 which is connected with the termination of the temporary artery (hyaloid) that 

 forms the continuation of the central artery of the retina through the vitreous 

 chamber. This vascular capsule of the crystalline lens forms the membrana 

 pupillaris (described on a subsequent page), and also attaches the borders of the 

 iris to the capsule of the lens. It disappears about the seventh month. 



The eyelids are formed at the end of the third month as small cutaneous folds, 

 which come together and unite in front of the globe and cornea. This union is 

 broken up and the eyelids separate before the end of foetal life. 



The lachrymal canal develops as a thickening of the epiblastic cells at the 

 bottom of the groove which extends upward toward the eye between the maxillary 

 and the fronto-nasal processes. The thickening becomes hollowed out into a 

 canal, and the lips of the groove meet over it, thus removing it from the surface. 



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 Drimary cerebral vesicle which eventually forms the medulla 

 oblongata, opposite the dorsal end of the second pharyngeal arch. The thicken- 

 ing is then followed by an involution of the epiblast, which becomes deeper and 

 deeper, sinking toward the base of the skull, and a flask-shaped cavity is formed ; 

 by the narrowing 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, which by its sinking inward 

 comes to be placed between the ali-sphenoid and basi-occipital matrices. From 

 it the internal ear is formed. The middle ear and the Eustachian tube are 

 developed from the remains of the first branchial cleft, while the pinna and 

 external meatus are developed from the soft parts overhanging the posterior mar- 

 gin of the same cleft. The primary otic vesicle becomes imbedded in a mass of 

 mesoblastic tissue, which rapidly undergoes chondrification and ossification. It, 

 as before stated, 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 prolongations or diverticula, from which the various parts of the 

 labyrinth are formed. One from the anterior end gradually elongates, and, form- 

 ing a tube bends on itself from left to right and becomes the cochlea. Three 

 others, which appear on the surface of the vesicle, form the semicircular canals. 

 Subsequently, a constriction takes place in the original vesicle, which, gradually 

 increasing, divides it into two, and from these are formed the utricle and 

 saccule. Finally, the auditory nerve, which has been developed from the 

 " neural crest " in the manner above described (page 122), pierces the auditory 

 capsule in two main divisions one for the vestibule, the other for the cochlea. 

 The middle ear and Eustachian tube are the remains of the first pharyngeal or 

 branchial cleft (hyo-mandibular), and are, from an early period, closed by the forma- 

 tion of the membrana tympani, which consists of a layer of epiblast externally, a 

 layer of hypoblast internally, and between the two of mesoblastic tissue consti- 



