No. 3.] A HUMAN EMBRYO TWENTY-SIX DAYS OLD. 465 



i.e. the future direction of the optic nerve. No doubt this layer 

 is identical with the peripheral veil as described by His. 1 



It is extremely difficult to locate the origin of the third 

 nerve. At the floor of the mid-brain there is a suspicious spot 

 composed of several dozen cells which are somewhat separated 

 from the remaining cells and lie partly within the terminal veil 

 (PI. XXX., Fig. 1, III.). No nerve fibres extend from the 

 brain into the surrounding tissue. 



The trochlear nerve is well marked as a small group of cells 

 in the ventral wall of the isthmus between the mid-brain and 

 hind-brain. The cells lie just under the terminal veil, and each 

 sends a single short pole towards the dorsal part of the brain. 

 They extend but half-way around the tube. 



The Gasserian ganglion with its three branches marks the 

 trifacial. Upon its ophthalmic branch a small group of cells indi- 

 cates the ciliary ganglion. From the Gasserian ganglion numer- 

 ous fibres enter the hind-brain as its sensory root. The 

 motor root arises more ventral from a large group of cells and 

 passes as a large bundle of fibres into the inferior maxillary 

 branch of the nerve. 



The sixth nerve is represented as a small group of cells, dor- 

 sal, but somewhat aboral from the first branchial cleft. None of 

 the cells send prolongations from the brain to form a distinct 

 nerve, but all of the unipolar motor cells are pointed in one 

 direction. 



It is extremely difficult to isolate the facial nerve from the 

 auditory nerve ganglion. Following it from the second bran- 

 chial arch it passes through the heart of the acoustico-facial 

 ganglion, and after entering the neural tube passes towards 

 the ventral side of the same, making an arch around the gan- 

 glion of the sixth to take its origin near the median line. 



The acoustic ganglion extends from the facial to the auditory 

 vesicle, to which it is adherent, and then with the facial nerve 

 sends twigs into the after-brain. The auditory vesicle is olive- 

 shaped, is placed at right angles to the after-brain, and from its 

 dorsal end there is a marked prolongation, the beginning of the 

 aquaeductus vestibuli. The walls are of quite even thickness 

 throughout, and the lumen is of the same general shape as the 

 vesicle. 



1 His u. Braune's Archiv, 18S9 (Randschleier). 



