THE BRAIN. 



257 



median diverticulum : this is at first directed slightly backwards, 

 but by the end of the third day becomes inclined forwards (Figs. 

 113 and 114, PN), and lies close beneath the external epiblast. 

 In the later stages, the pineal body increases in size, becomes 

 dilated at its distal end, and gives off a number of branching 



MEL 



V BL 



BM 



jir 



KY 



OH 



LP 



LA 



FIG. 115. A Chick Embryo at the end of the fifth day of incubation, seen from 

 the right side. The amnion has been removed, x 20. 



BL, cerebellum. BM, optic lobe, formed from the mid-brain. El, auditory 

 vesicle. HY, hyoid arch. LA, fore-limb or wing. LP, hind-limb or leg. NE, 

 ganglion of first spinal nerve. ]STM, commissure connecting first and second spinal 

 ganglia. OC, eye. OH, choroidal fissure. BV, ventricle of heart. TA, allantois. 

 YK, yolk-stalk, cut short. I, olfactory nerve. Ill, third nerve, or motor oculi. V, 

 fifth or trigeminal nerve. V, ophthalmic branch of trigeminal nerve. VII, seventh or 

 facial nerve. VIII, eighth or auditory nerve. IX, ninth or glossopharyngeal nerve. 

 X, tenth or pneumogastric nerve. X', visceral branch of pneumogastric nerve. X'', 

 commissure connecting pneumogastric nerve with the ganglion of the first spinal nerve. 



tubular diverticula. Its condition on the eighth day is shown in 

 Fig. 116, PN. 



In front of the pineal body the roof of the thalamencephalon 

 is very thin, and becomes thrown into folds which hang down 

 into the ventricle : between the layers of these folds numerous 



s 



