ORGANS OF VISION OF THE VERTEBRATA. 499 



investment round the whole eye ; and there is no differentiated sclerotic and 

 only an imperfect choroid. 



In a still later stage a distinct mesoblastic element for the iris is formed. 

 When the Ammoccete is becoming a Lamprey, the eye approaches the 

 surface ; an anterior chamber is established ; and the eye differs from that 

 of the higher types mainly in the fact that the cornea is hardly distinguished 

 from the remainder of the skin, and that a sclerotic is very imperfectly 

 represented. 



Optic vesicles. The development of the primitive optic vesicles, so 

 far as is known, is very constant throughout the Vertebrata. In Teleostei 

 and Lepidosteus alone is there an important deviation from the ordinary 

 type, dependent however upon the mode of formation of the medullary keel, 

 the optic vesicles arising while the medullary keel is still solid, and being at 

 first also solid. They subsequently acquire a lumen and undergo the 

 ordinary changes. 



The lens. In the majority of groups, viz. Elasmobranchii, Reptilia, 

 Aves, and Mammalia, the lens is formed by an open invagination of the 

 epiblast, but in Amphibia, Teleostei and Lepidosteus, where the nervous 



FIG. 292. EYE OF AN AMMOCCETES LYING BENEATH THE SKIN. 



ep. epidermis; d.c. dermal connective tissue continuous with the sub-dermal 

 connective tissue (s.d.c), which is also shaded. There is no definite boundary to this 

 tissue where it surrounds the eye. 



m. muscles ; dm. membrane of Descemet ; /. lens ; v.h. vitreous humour ; r. retina ; 

 rp. retinal pigment. 



layer of the skin is early established, this layer alone takes part in the 

 formation of the lens (fig. 293, /). The lens is however formed even in 

 these types as a hollow body by an invagination ; but its opening remains 

 permanently shut off from communication with the exterior by the epidermic 



322 



