152 



BLIND VERTEBRATES AND THELR EYES. 



In a larva 5 mm. long the eye is still in contact with the epidermis on one side 

 and the incipient dura mater on the other. The epidermis is distinctly thinner 

 over the eye, reaching an extreme thinness of 16 fi as compared with a thickness 

 of 40 ju, at a distance of 100 fi below the eye and of 0.24 /a at 100 jx above the eye. 



The lens lies directly beneath the skin. In this particular eye (fig. 62 a) it is an 

 ellipsoid, 30 /x by 38 ti (36 by 28 in another eye). It is entirely separated from the 

 skin and takes on a deeper stain. The cells of the lens are not very regularly grouped, 

 but apparently they are arranged about a median point or space. The lens lies 

 entirely outside of the eye in contact with the outer face of the dorsal part of the 

 iris. The eye proper is a subspherical solid mass with only a shallow depression 

 below the lens representing the vitreous cavity and choroid fissure. In the eye 

 more particularly described here the depression is filled largely with blood corpuscles 

 (fig. 62 a, cpl.sng.). 



The pigmented layer is not more than 4 fi thick, and is very sparingly pigmented 

 over the posterior face of the eye. At the iris and the lower margin of the choroid 

 fissure it is continuous with the inner layers of the retina through cells whose nuclei 

 are distinctly elongate. The retina proper, from the pigmented layer to the vit- 

 reous cavity, is 64 /1 thick. 



cplstig. 



tkpig. 



Ins. 



e'th.pig- 



fis.chd. cpisng- 



did. 



Fig. 61. Two Vertical Sections of Eye of Individual about 5 mm. long. Fig. a taken through Lens, Vitreous 

 Cavity, and Choroid Fissure. Fig. 6, Second Section Proximal to that from which tig. 61 a was drawn and 

 passes through Innermost Part of Vitreous Body. Layers of Retina have not yet begun to be differentiated. 



It is differentiated into a nuclear layer (the outer and inner together) and the 

 ganglionic layer, separated by the incomplete inner reticular layer. The ganglionic 

 layer is composed of two sorts of cells. Those nearer the vitreous cavity have much 

 more distinct nucleoli than those nearer the reticular layer. Cell multiplication 

 is still going on. 



The optic nerve is well developed, forming a solid strand of fibers, 12 /a in diam- 

 eter, readily traceable to the brain. 



The muscles are represented by strands of cells closely crowded. No striation 

 is evident. 



SIX-MILLIMETER STAGES. 



In embryos 6 mm. long the cells giving rise to the oblique muscles and those 

 for at least 2 of the recti can be distinguished. Scleral cartilages are not yet formed. 



In 3 of the specimens sectioned there was no indication of a lens. In others it 

 was well developed. Cell division was still going on in the retina. 



The optic vesicle was very shallow. The rim of the vesicle was wide and still 

 continuous with the choroid fissure, which showed as a shallow groove along the 

 ventral surface. The choroid fissure, instead of leading into a central secondary 



