THE ECTODERM AND ITS DERIVATIVES 



183 



I. Olfactory: This is sensory, arising after hatching (11 mm.) 

 from neuroblasts of the olfactory lobe (telencephalon) and 

 growing to the olfactory placode. 

 II. Optic: This is sensory, arising at an early stage (6 mm.) from 

 the neuroblasts of the retinal layer of the optic cup to grow 

 along the ventral wall of the optic stalk to join the lateral 

 wall of the diencephalon. 



III. Oculomotor: This is motor, arising Just before hatching from 

 the ventro-lateral wall of the mesencephalon (crura cerebri) 

 to innervate four muscles of the eye. These are the rectus 

 superior and the ciliary ganglion, the rectus inferior, the 

 rectus medialis, and the obliquus inferior. 



IV. Trochlear: This is motor and remains very small. It arises late 

 in development from near the roof of the mesencephalon be- 

 tween the optic lobes and the cerebellum just posterior to the 

 valvula cerebelli and the posterior commissure. It innervates 

 the superior oblique muscles of the eye. 



V. Trigeminal: This is a mixed nerve, also known as the trifacial 

 ganglion, which appears as early as the 5 mm. stage and has 

 a complicated origin from the dorsal (superficial) ganglionic 

 elements of the first cranial crest segment and the ganglionic 



(Courtesy, Jenkinson: "Vertebrate Embryology," Oxford, The Clarendon Press.) 



