t504 PHYSIOLOGICAL DEVELOPMENT. 



columns, everj' one of wticli is itself a perfect eye, are 

 contained, is closed posteriorly by a membrane, which, 

 appears to be neither more nor less than the middle 

 tegumentary membrane, pierced for the passage of the 

 optic nerve ; so that the ocular chamber at large 

 results from the separation at a point of the two external 

 layers of the general envelope." Thus too is 



it, in the main, even with the highly-developed eyes of 

 the Vertebrata. " The three pairs of sensory organs apper- 

 taining to the higher senses," says Prof. Huxley — " the nasal 

 sacs, the eyes, and the ears — arise as simple coeoal involutions 

 of the external integument of the head of the embryo. 

 That such is the case, so far as the olfactory sacs are con- 

 cerned, is obvious, and it is not difiBcult to observe that the 

 lens and the anterior chamber of the eye are produced in a 

 perfectly similar manner. It is not so easy to see that the 

 the labj'rintli of the ear arises in this way, as the sac resulting 

 from the involution of the integument is small, and remains 

 open but a very short time. But I have so frequently veri- 

 fied Huschke's and Remak's statement that it does so arise, 

 that I entertain no doubt whatever of the fact. The outer 

 ends of the olfactory sacs remain open, but those of the 

 ocular and auditory sacs rapidly close up, and shut off their 

 contents from all direct communication with the exterior." 

 So that, marvellous as the fact appears, all that part of the 

 eye which lies between its outer surface and the back of the 

 crystalline lens, ia formed in the same way as an ordinary hair- 

 sac, and is composed of homologous parts. The interior coat is 

 the epidermic layer, originally continuous with the surface of 

 the skin ; and only made discontinuous with it by closure of 

 the sac at the point which is afterwards the centre of the 

 cornea. This cornea, or front wall of the chamber thus shut 

 off, is consequently composed of a doubled epidermic laver 

 and an intermediate layer of the derma included in the fold 

 of the integument. The crystalline lens, lying at the far side 

 of this chamber, is simply a thickening of the epidermic layer 



