DEVELOPMENT OF THE EAR. 89 



Accessory structures. The eyelids make their appearance gradually as folds 

 of integument, subsequently to the formation of the eyeball (fig. 103). About the 

 third month of foetal life the two folds, one forming the upper and the other the 

 lower lid, meet and unite by a growth together of the epithelium at the margins of 

 the folds, so as to cut off the conjunctival sac from the exterior. A short time 

 before birth they again become disunited. 



A third fold (of the conjunctiva) appears at the inner canthus, and in many 

 vertebrates developes into a well-marked third eyelid, the membrana niditans. In 

 man it remains rudimentary, forming the plica semilunaris. 



The glands, hairs, and other structures belonging to the eyelids, are deve- 

 loped in the same way as the corresponding structures in the rest of the 

 integument. 



The lachrymal gland is developed in the third month as a number of out- 

 growths from the deeper layer of the epithelium, at the upper and outer part of the 

 conjunctival sac. The outgrowths are at first solid, and branch into the surrounding 

 connective tissue as with other racemose glands, subsequently becoming hollowed 

 out and differentiated into ducts and acini. 



The lachrymal canals and ducts are usually described as being directly 

 developed by the enclosure of the fissure which separates the lateral nasal process 

 from the maxillary process (see Development of Nose, p. 95, and figs. Ill, 112), 

 and which passes in the early embryo from the eye to the upper part of the naso- 

 buccal cavity (lachrymal fissure). But it has been shown, chiefly by the researches 

 of Born, that in most animals the canal is at first formed as a thickening of the 

 rete mucosum of the epidermis, which sinks into the corium along the line of that 

 fissure. The thickening subsequently becomes separated from the rest of the 

 epidermis, and hollowed out to form an epithelial tube, which leads from the 

 conjunctiva into the nasal cavity. 



The bifurcation of the duct where it opens on the conjunctiva is produced, according to 

 Ewetsky, by a broadening out of the epithelial cord at the inner canthus, and its subsequent 

 separation into two parts by an ingrowth of connective tissue in its middle, the two parts 

 developing into the upper and lower lachrymal canals. 



DEVELOPMENT OF THE EAR 



The essential part of the ear, viz., the epithelial lining of the labyrinth, is 

 developed in much the same way as the crystalline lens, as an invagination of the 

 external epiblast, which at first appears as a pit of thickened epithelium (auditory 

 pit, fig. 104, A.), but is gradually converted by a growing together of the margins of 

 the pit into a hollow island of epiblast, the auditory or otic vesicle (fig. 104, B). 

 This process occurs somewhat after the formation of the eye is laid, and at quite a 

 different part of the head, viz., on either side of the hind-brain just over the upper 

 end of the first post-oral visceral cleft. The vesicle comes at first into close contact 

 with the hind-brain, except where the ganglionic rudiment of the auditory nerve 

 projects between them, but it subsequently becomes entirely surrounded by meso- 

 blast, which separates it from both the neural and external epiblast. 



The hind-brain does not send out a hollow process towards the otic vesicle 

 corresponding to the optic processes of the fore-brain, but the auditory nerve 

 developes from a solid outgrowth of the neural crest in the same way as the 

 posterior roots of the spinal nerves and parts of many other of the cranial nerves 

 (see p. 78). 



The otic vesicle is at first flask-shaped, with the somewhat elongated mouth of 

 the flask directed externally towards the original point of connection with the 



