362 



THE CAT. 



[chap. X. 



formed from a growth of mesoblast intervening between tlie 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 



A 



Fig. 102.— Section of the commencing Eye of an Embrvo, in three stacks. 



A. Commencement of the formation of tlie lens 



by (lepression of :i ]i;irt of C, the epiblast. 



V''. Tlie piiiiiitive ocular vesicle or 



iier\'ous out,i;rowtli from the brain, 



now (loubleil back on itself liy the 



depression of theeomnieuciuglens (I). 



B. Tlie lens depression has become' enclosed, 



and the lens itself is beginning to bo formed 



within it. The optic vesicle lias here 

 liecome more folded back. 

 A third sta^e, in which the secondaiy optic 

 vesicle— the upgrowth foi'ming the vitreims 

 humour (r)— l>egins to be formeil. The 

 primitive cavity of the cerebral optic 

 vesicle (pi) is liere reduced to a chink by 

 the still further infolding of that vesicle. 



the eyeball dcvclopes 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 EAit, 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 con-esponding 

 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 ccrebro-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 rcsictc. This 

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

 thelium of that hibyrlnth which contains the cndoljanph. All the 

 structures external to-fhi.>^, namely, the fibrous structure of the 

 membranous labyrinth, the peril^inph, and the solid structures which 



