THE NERVOUS SYSTEM 265 
nerve is entirely intramedullary, the retina being part of the 
wall of the embryonic brain; its development will therefore be 
considered in connection with the development of the eye. 
Fic. 156. — Sagittal section through the head of a chick embryo of 5 days, 
showing the floor of fore-brain, olfactory pit, and developing olfactory 
nerve between. (After Disse.) 
a., Unipolar neuroblasts near the olfactory epithelium. b., Bipolar cell 
in the olfactory nerve. c¢., Unipolar cell near the brain. F. B., Floor of 
fore-brain. N’bl., Neuroblast in the olfactory epithelium. olf. Ep., Olfac- 
tory epithelium. olf. N., Olfactory nerve. olf. P., Cavity of olfactory pit. 
D) 
3. The third cranial or oculo-motor nerve arises from a group 
of neuroblasts in the ventral zone of the mid-brain near the median 
line, and appears external to the wall of the brain at about sixty 
hours (about 28-30 somites). At this time it appears as a small 
group of axones emerging from the region of the plica encephali 
