LATER DEVELOPMENT OF CHICK AND RABBIT 431 



large pit. The edges of this come together, and finally it becomes 

 cut off from the superficial ectoderm as a thick-walled spherical 

 vesicle, that almost fills the opening of the optic cup by about the 

 end of the third day. The inside thickened layer of the optic cup 

 is the beginning of the sensitive layer of the retina, while the outside 

 part is the pigment layer, and pigment is formed in. it during the 

 fourth day. 



It has been noted that not only is the original outer wall of the 

 primary vesicle involved in the invagination, but also the ventral 

 wall back to the optic stalk. Thus it comes about that when the 

 optic cup is first formed it is incomplete on the ventral side. The 

 adjacent regions expand and fill in this gap to a certain extent, but 



Lens p-Cfi 



Lj, 



Lf/B. 



FIG. 151. -Transverse section through the eyes and heart of an embryo of 

 about thirty-five pairs of somites. From Lillie. 



ch.Fis., choroid fissure ; D.C., duct of Cuvier ; Lg., lung ; pl.gr., pleural groove ; V.c., posterior 

 cardinal vein ; Y.S., yolk-sac ; Ao., aorta ; Chor., chorion ; Atr., atrium ; Dienc., diencephalon; 

 p.C., parietal cavity ; p. Ch., posterior chamber of the eye ; am., amnion; B.A., bulbus arteriosus. 



for quite a while a narrow cleft, termed the choroid fissure, is left 

 running from the rim of the cup back to the optic stalk. The 

 presence of this fissure is of considerable importance to the future 

 course of development of the eye. In the first place it leaves the 

 whole of the inside layer of the retina in communication with the 

 stalk, and so when the nerve cells of the inside of this layer send out 

 their axons these can grow back along the stalk as the optic tract 

 into the brain. This leads to the disappearance of the cavity of the 

 stalk, and its transformation into the definitive optic nerve. Secondly, 

 the fissure allows that part of the adjacent mesoderm from which 

 the vitreous humour will be derived to spread into the optic cup, 

 and it also allows for the ingrowth of the retinal arteries and 

 veins. 



