ANATOMY. 



483 



the brain becomes the basis of the optic ' nerve,' and along 

 its tract nerve-fibres grow out from the retinal ganglion-cells 

 to the brain. 



The optic * nerve ' is therefore not a nerve but a tract 

 leading from the peripheral brain ganglion-cells of the retina 

 to the visual centres. 



Fig. 91. Transverse section across the head of a Chick Embryo at the 

 forty-eighth hour of incubation. The section is taken along a line 

 corresponding to one joining the relerencc letters El and OL. in the 

 three -day embryo shown in fig. 94. Owing to the cranial flexure, 

 both fore- brain and hind- brain are cut by the section. The right side 

 of the section is slightly anterior in position to the left side. x 60. 



A, aorta. AC, carotid artery. BF, cavity of fore-brain. BH, cavity of 

 hind-brain. CH, notochord. El, auditory pit. HM, nyo-mandil>ular cleft. 

 MN, mandibular arch. OC, cavity of optic cup. OL, inva^inat on of epibla*t 

 to form the lens. OS, optic stalk. PT, pituitary body. TP, pharynx. VII, 

 facial nerve. 



The further development of the eye takes place gradually. 

 The rods and cones are the last parts to be formed. 



