190 THE GERM LAYER DERIVATIVES 



Summary of Embryonic Development of the 11 mm. Frog Tadpole: 

 Ectodermal Derivatives 



Epidermis — thickened, ciliated on tail only. 



Central Nervous System — differentiated into four brain vesicles and spinal 

 cord. 

 Prosencephalon (Forebrain) 



Telencephalon — anterior to optic recess; thin-roofed. 



Cerebral hemispheres develop around each of lateral ventricles. 

 Anterior choroid plexus as median dorsal invagination into telocoel 



which becomes vascularized. 

 Olfactory (I) nerve from floor of cerebral hemisphere to mesial side 

 of nasal tube. 

 Diencephalon — from epiphysis to dorsal thickening to tuberculum 

 posterius. 

 Epiphysis as mid-dorsal evagination just posterior to choroid plexus. 

 Optic recess — just posterior to torus transversus or region of lamina 

 terminalis (anterior fusion of neural folds) as median ventral evagina- 

 tion. 

 Optic chiasma — thickened floor where optic nerve enters (posterior to 



recess). 

 Optic (II) nerve — from chiasma to eyes, may be seen entering retina. 

 Infundibulum — bulbous evagination in floor of diencephalon between 

 optic chiasma and tuberculum posterius, ventral to notochord. 

 Mesencephalon — bounded by dorsal thickening and tuberculum posterius. 

 Optic lobes from dorsal thickening, bi-lobed, become corpora bigemina. 

 Cavity — mesocoel, iter, or aqueduct of Sylvius. 

 Oculomotor (III) nerve — mesio-lateral, from floor. 



Trochlear (IV) nerve — dorso-lateral, just posterior to optic lobes and diffi- 

 cult to find, being small. 

 Rhombencephalon — from dorsal thickening to spinal cord, metencephalon and 

 myelencephalon not distinguishable in the frog. 

 Cerebellum — from thickened roof, anterior. 

 Posterior choroid plexus — from thin, vascular posterior roof. 

 Otic vesicles — found at level of cerebellum. 



Cranial nerves V to X are associated with this portion of the brain. 

 Trigeminal (V) nerve — anterior to otocyst, large, sends branches to 



mandibular and maxillary processes of first visceral arch. 

 Abducens (VI) nerve — from floor of rhombencephalon, anterior and 

 ventral to origin of trigeminal. Supplies eye muscles as do oculomotor 

 and trochlear. 

 Facial (VII) and auditory (VIII) nerve — arises as a single ganglion, mesio- 

 posterior to otocyst, supplying facial muscles and saccule and utricle 

 of ear. 

 Glossopharyngeal (IX) nerve — arises posterior to otocyst, sending 

 branches to first branchial arch. Ganglion. 



