ORGANS OF VISION OF THE VERTEBRATA. 



501 



conspicuous by the absence of pigment, and at a later period by the deep 

 colour of its pigment. Finally, a little after the ninth clay, no trace of it is 

 to be seen. 



Up to the eighth day the pecten remains as a simple lamina ; by the 

 tenth or twelfth day it begins to be folded or rather puckered, and by the 

 seventeenth or eighteenth day it is richly pigmented and the puckerings 



fb 



FIG. 293. SECTION THROUGH THE FRONT PART OF THE HEAD OF A LEPIDOS- 



TEUS EMBRYO ON THE SEVENTH DAY AFTER IMPREGNATION. 

 al. alimentary tract ; fb. thalamencephalon ; /. lens of eye; op.v. optic vesicle. 

 The mesoblast is not represented. 



have become nearly as numerous as in the adult, there being in all seventeen 

 or eighteen. The pecten is almost entirely composed of vascular coils, 

 which are supported by a sparse pigmented connective tissue ; and in the 

 adult the pecten is still extremely vascular. The original artery which 

 became enveloped at the formation of the pecten continues, when the latter 

 becomes vascular, to supply it with blood. The vein is practically a fresh 

 development after the atrophy of the distal portion of the primitive vascular 

 loop of the vitreous humour. 



There are no true retinal blood-vessels. 



In the formation of the optic cup the extreme peripheral part of the optic 

 nerve, which is in immediate proximity with the artery of the pecten, 

 becomes folded. The permanent opening in the choroid fissure for the 

 pecten is intimately related to the entrance of the optic nerve into the 

 eyeball ; the fibres of the optic nerve passing in at the inner border of the 

 pecten, coursing along its sides to its outer border, and radiating from it as 

 from a centre to all parts of the retina. 



In the Lizard the choroid slit closes considerably earlier than in the 

 Fowl. The vascular loop in the vitreous humour is however more developed. 

 The pecten long remains without vessels, and does not in fact become at all 



