ORGANS OF THE SENSES AND THE COMMON INTEGUMENT 



and extends in the form of a groove for some distance along the optic stalk, so that, 

 for a time, a gap or fissure, the choroidal fissure, exists in the lower part of the 

 cup (Fig. 865). Through the groove and fissure the mesoderm extends into the 

 optic stalk and cup, and in this mesoderm a bloodvessel is developed; during the 

 seventh week the groove and fissure are closed and the vessel forms the central 

 artery of the retina. Sometimes the choroidal fissure persists, and when this 

 occurs the choroid and iris in the region of the fissure remain undeveloped, giving 

 rise to the condition known as coloboma of the choroid or iris. 



TelencepJialon 





Thalamencephalon 



Optic stalk 



Edge of optic cup 



Choroidal fissure 



Arteria centralis 

 retinae 



Fio. 865. Optic cup and choroidal fissure seen from below, from a human embryo of about four weeks. 



(Kollmann.) 



The retina is developed from the optic cup. The outer stratum of the cup 

 persists as a single layer of cells which assume a columnar shape, acquire pigment, 

 and form the pigmented layer of the retina; the pigment first appears in the cells 

 near the edge of the cup. The cells of the inner stratum proliferate and form a 

 layer of considerable thickness from which the nervous elements and the susten- 

 tacular fibers of the retina, together with a portion of the vitreous body, are 

 developed. In that portion of the cup which overlaps the lens the inner stratum is 

 not differentiated into nervous elements, but forms a layer of columnar cells which 

 is applied to the pigmented layer, and these two strata form the pars ciliaris and 

 pars iridica retinae. 



The cells of the inner or retinal layer of the optic cup become differentiated into spongioblasts 

 and germinal cells, and the latter by their subdivisions give rise to neuroblasts. From the spongio- 

 blasts the sustentacular fibers of Muller, the outer and inner limiting membranes, together with 

 the groundwork of the molecular layers of the retina are formed. The neuroblasts become 

 arranged to form the ganglionic and nuclear layers. The layer of rods and cones is first developed 

 in the central part of the optic cup, and from there gradually extends toward the cup margin. 

 All the layers of the retina are completed by the eighth month of fetal life. 



The optic stalk is converted into the optic nerve by the obliteration of its cavity 

 and the growth of nerve fibers into it. Most of these fibers are centripetal, and 

 grow backward into the optic stalk from the nerve cells of the retina, but a few 

 extend in the opposite direction and are derived from nerve cells in the brain. The 

 fibers of the optic nerve receive their medullary sheaths about the tenth week after 

 birth. The optic chiasma is formed by the meeting and partial decussation of the 

 fibers of the two optic nerves. Behind the chiasma the fibers grow backward as 

 the optic tracts to the thalami and mid-brain. 



The crystalline lens is developed from the lens vesicle, which recedes within the 



