THE CRANIAL NERVES OP SCYLLIUM. 101 



the top of the thickened side of the hind-brain just before its junction 

 with the thin roof ; from this origin the root runs downward, alongside 

 of and closely applied to the brain, but unconnected with it, to join 

 the secondary root (vn /3). This latter is now situated still nearer 

 to the ventral surface than at its first appearance, the distance between 

 the two roots being considerably greater than at the earlier stages, as 

 is evident from a comparison of fig. 9 with fig. 6. The two roots also 

 differ histologically, the dorsal or primary root consisting almost entirely 

 of elongated fusiform cells, whilst the ventral root (vii /3) is mainly 

 composed of spherical ganglion cells. 



This ventral root, at stage o, has, as shown in the figure, two 

 distinct attachments to the brain, one just below the other. It is 

 shown in longitudinal and vertical section at stage n in fig. 11, where 

 the brain presents opposite to its point of origin an external bulging 

 precisely similar to that opposite to the secondary root of the fifth 

 (v /3). The dorsal or primary root (vn a) is shown at the same stage 

 in fig. 12. 



In fig. 14 the two roots of the seventh are seen in longitudinal and 

 vertical section, at an age intermediate between stages o and p. The 

 dorsal root (vn a) arises very far up the side of the brain, in fact, as 

 in the earlier stages, from the junction of side and roof; it is of con- 

 siderable length, is widely separated from the secondary root, and still 

 consists mainly of fusiform cells ; the secondary or ventral root, which 

 is overlaid and almost concealed by the primary root, is only seen 

 in part, its most anterior portion alone being visible. 



The dorsal or primary root is also well shown at the same stage in 

 fig. 15. 



B. Comparison of the emhryonic roots of the fifth and seventh nerves 

 with those of the adult. — It will be convenient here to briefly summarise 

 our results concerning the roots of origin of the fifth and seventh 

 nerves, and to trace their changes up to the adult form. 



About the close of stage i the fifth nerve (fig. 1) still retains its 

 primary attachment (v a) to the brain, and is on the point of acquiring 

 its secondary one (v j3) ; owing to the growth of the roof of the brain 

 the two primary roots, which were at first continuous across the top 

 of the brain, are now widely separate. The seventh nerve (fig. 2) 

 arises by its primary root from the dorsal summit of the brain, whose 

 roof at this point has not yet commenced its rapid growth, so that the 



