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. 



Fig. 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. 



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 



