362 



THE CAT. 



[CHAP. x. 



formed from a growth of mesoblast intervening between the incipient 

 lens and the epiblast. From the margins of the chamber of the 

 aqueous humour, a growth extends inwards on all sides which 

 becomes the iris, and divides the chamber into two portions. The 

 vitreous humour is formed by a growth of mesoblastic tissue up 

 through a fissure left below during the infolding of the primary optic 

 vesicle and formation of the optic cup. This fissure gets gradually 

 closed up, though traces of it, called the ocular cleft, may be discerned 

 for a considerable time. The mesoblastic tissue around the eyeball 

 becomes condensed into the sclerotic. The external skin in front of 



Fig. 162. SECTION OF THE COMMENCING EYE OF AN EMBRYO, IN THREE STAGES. 



A. Commencement of the formation of the lens 



by depression of a part of C, the epiblast. 



pr. The primitive ocular vesicle or 



nervous outgrowth from the brain, 



now doubled back on itself by the 



depression of the commencing lens (I). 



B. The lens depression has become^ enclosed, 



and the lens itself is beginning to be formed 



within it. The optic vesicle has here 

 become, more folded back. 



C. A third stage, in which the secondary optic 

 vesicle the upgrowth forming the vitreous 

 humour (v) begins to be formed. The 

 primitive cavity of the cerebral optic 

 vesicle (pr) is here reduced to a chink by 

 the still further infolding of that vesicle. 



the eyeball developes a fold of membrane above and below. These 

 increase in size, and become the eyelids, their inner lining and the 

 epiblast coating the cornua being transformed into the conjunctiva. 

 The eyelids, when formed, become glued together at their margins 

 till nine days after birth. The conjunctiva is continuous with the 

 lining of the lachrymal canal, which latter is a persistent remnant 

 of the fissure, at first wide, which primitively exists between the 

 frontal and the maxillary processes of the embryo. 



The DEVELOPMENT OF THE EAR, in so far as it arises by involu- 

 tion of the epiblast, resembles that of the eye, but it differs from it 

 greatly in that there is no outgrowth from the brain corresponding 

 with such epiblastic involution. The first appearance of the future 

 internal ear takes place (at a very early period) on each side of the 

 hind-brain, when an involution of the epiblast forms a pit extending 

 down into the mesoblast, which lies externally to the cerebro-spinal 

 axis of that region. This pit deepens, and its margins close over 

 and unite, so forming it into a closed sac called the otic vesicle. This 

 vesicle becomes the internal labyrinth. The epiblast forms the endo- 

 thelium of that labyrinth which contains the endolymph. All the 

 structures external to this, namely, the fibrous structure of the 

 membranous labyrinth, the perilymph, and the solid structures which 



