136 EMBRYOLOGY OF THE LOWER VERTEBRATES ch. 



epithelium — and a highly vascular and deeply pigmented choroid 

 between the sclerotic and the pigment-layer of the retina. 



As development proceeds watery fluid collects in the space 

 between the lens and the cornea. Round its periphery this space 

 is bounded internally by the inturned edge of the optic cup 

 which overlaps the lens. For a considerable distance out from 

 the lens, as far as the ora serrata, this portion of the wall of the 

 optic cup does not undergo differentiation into Functional retinal 

 tissue. Its marginal portion covered on its outer surface by a 

 prolongation of choroid becomes the iris. Here the inner "retinal" 

 layer develops pigment and becomes in other respects like the outer 

 layer. The outer layer itself gives rise to the radial and circular 

 muscle fibres of the iris — a fact of morphological interest to be 

 grouped along with the development of muscle fibres by the 



Pig. 75. — Transverse sections through the brain of Lepidosiren, showing the solid 

 optic rudiments (o.r). 



A, stage 21 ; B, stage 23. 



ectoderm of flask-glands referred to on a previous page. Outside 

 the periphery of the iris the retinal layer remains comparatively 

 undifferentiated as an unpigmented layer of columnar epithelium. 

 It lies internal to a special development of choroid which forms 

 the ciliary body and in which are developed the muscles of 

 accommodation. 



The mode of origin of the eye as a whole and of its component 

 parts as seen in the Fowl embryo having been sketched in outline, it 

 will now be convenient to take the various parts in turn and note 

 further features of interest without restricting consideration merely 

 to the Bird. 



In the first place it is to be noted that the rudiment of the eye 

 is at first solid in those Vertebrates in which the central nervous 

 system is solid at the time of its appearance, the cavity of the optic 

 rudiment developing secondarily (Fig. 75, A, B, o.r). As this 

 secondary cavity increases in size both in the optic rudiment and in 

 the brain a condition is gradually reached like that already described 

 and the further development is on normal lines. 



