No. i.] MORPHOLOGY OF VERTEBRATE HEAD 99 



geminus und Acustico-facialis besitzt," "vor allem " surely can- 

 not mean that we are to hold by the above dictum even though 

 it be contradicted by facts. 



The peculiar development of the trochlearis led me to look 

 with great interest for the origin of the oculomotorius. This 

 nerve arises in the stage when three branchial clefts have 

 appeared, or when the length of the embryo is about 1 cm. 

 At first the nerve lies in a plane nearly horizontal, and ex- 

 tends from the inner and dorsal angle of the ciliary ganglion to 

 the brain. Its path crosses that of the veins c 2 , Fig. 10, PI. IV, 

 and lies from two to four sections (3-^ to -^ mm. in thickness) 

 above these vessels. Horizontal sections through the venous 

 space aa ', Fig. 10, pass above the nerve ; sections through the 

 branches, c 2 , Fig. 10, pass below it. Hence the region to be 

 searched for the origin of the oculomotorius is limited. Sec. 2, 

 PI. V, shows the oculomotorius when it consists of but one cell, 

 extending from the ciliary ganglion to the brain. To judge 

 from the position of this cell, it might have migrated either 

 from the brain or from the ganglion, although the appearance 

 of the inner and dorsal angle of the ganglion indicates that a 

 migration of other cells may be about to take place. I was so 

 fortunate as to obtain another series of sections in which the 

 oculomotorius is also represented by but one cell (sec. 1, PI. V) 

 and in which the position of the cell leaves no doubt that its 

 migration from the ciliary ganglion has recently taken place. 

 In sec. 3, PI. V, at a stage when four branchial clefts are through, 

 the nerve is well established. These three sections tell together 

 the story of the origin of the oculomotorius, or at least trace its 

 source back to the inner cells of the ciliary ganglion. Whence 

 these cells are derived, I am unable to say, and upon the answer 

 to this question depends the true position of the oculomotorius 

 in the series of segmental nerves. The " Anlage " of the ciliary 

 ganglion lies in the region where the prolongation from the 

 neural crest that forms the thalamic nerve meets that from 

 which the trochlear and trigeminal nerves arise. Later, when 

 the ciliary ganglion is well developed, the connection with the 

 thalamic nerve is lost, and the ophthalmicus profundus is seen 

 to unite the ciliary ganglion with the Gasserian at the point 

 where the primary trochlearis enters. Three cranial nerves are 

 therefore intimately connected with the formation of the ciliary 



