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develop as lateral oiilgrowlhs from the caudal end of the pn)sencephaIon 

 (future diencephalon) (lig. 354I3-D). These outgrowths, the primary optic 

 cvaj;>inati(>iis, begin to appear early on the second day of incubation, even 

 liefore the neural tube is closed. At about the 12 somite stage, which exists 

 at about 38 hours of incubation, the primary optic vesicles begin to constrict 

 proximally in the area near the brain, and distally they come into conatct 

 with the overlying epidermis (fig. 36()B). At 16 somites, or about 45 to 49 

 hrs. of incubation, the primary optic vesicle has dilTerenliated into a proximal 

 constricted optic .stalk and a distal primary optic vesicle (lig. 36()C). At the 

 22 somite stage (about 50 hrs. of incubation), the Optic vesicle begins to 

 invaginate and the overlying ectoderm starts to thicken preparatory to for- 

 mation of the lens (fig. 360D). At 55 hrs. of incubation, invagination of the 

 optic vesicle is completed, and the two-layered or secondary optic vesicle is 

 formed. The lens rudiment at this time is an invaginated vesicle still retaining 

 a small, open duct to the external surface. The following features of eye 

 development in the 55 hr. chick are present: 



( 1 ) The lens vesicle is almost completely formed. 



(2) The secondary optic vesicle is in the form of a cup, who.se inner layer 

 forms the retinal rndinient, and its outer layer the rudiment of the 

 pigmented coat of the eye. 



(3) The ventral or lower edge of the optic stalk also is invaginated to form 

 the choroid fissure, which continues the invagination of the optic cup 

 back into the region of the ventral area of the optic stalk (fig. 36()E). 



In the 72 to 75 hr. chick (about 40 pairs of somites) the two-layered optic 

 cup presents an outer thinner layer, the rudiment of the pigmented coat, and 

 an inner, thicker retinal layer. I'he lens vesicle at this time is completely free 

 from the overlying ectoderm and its inner (medial) wall is thicker than the 

 external wall. The medial thicker wall is the rudiment of the body of the len.s 

 and the outer thinner wall is the anterior epithelium of the lens (fig. 360F). 

 At 96 to 100 hrs. of incubation the developing lens of the eye has undergone 

 marked changes from the condition present at 72 to 75 hrs. of incubation. 

 The medial wall of the lens vesicle has thickened greatly and lens fibers are 

 evident, while the lateral wall of the vesicle forms a relatively thin epithelial 

 membrane (lig. 360(i). The mesoderm below the ectoderm also forms a thin, 

 internal epithelial membrane which lines the developing cornea. At this time 

 the lips of the optic cup show the first indications of two distinct areas, viz.: 

 a retinal or optic part, the pars optica retinae, which forms the visual portion 

 of the adult retina, and a pars caeca retinae lying distally in the region of the 

 lens (fig. 360G). The pars caeca does not develop visual cells. At the eighth 

 to ninth days of incubation, the pars caeca shows the beginning stages of ciliary 

 body formation, and the development of the iris (fig. 360H). The mesenchyme 

 overlying the iris forms the condensed stromal tissue, but the sphincter and 



