THE CRANIAL NERVES OP SCYLLIUM. 93 



again subdivide near their terminations (figs. 11 and 15), but all the 

 branches, whether primary or secondary, are distributed to the stiperior 

 oblique muscle, with the possible exception of a few of the smaller 

 ones, which appear to join the ophthalmic branch of the fifth. We 

 would suggest that this branching may possibly be an indication of 

 the fourth nerve having previously had a more extended distribution 

 than its present very limited one. 



On the whole, our observations lead us to believe that the fourth 

 nerve is to be regarded as a separated part of that segmental nerve of 

 which the third nerve forms the main portion. A further suggestion 

 concerning the fourth nerve will be made after the seventh nerve has 

 been considered. 



The Fifth (Trigeminal) Nerve. —We propose to consider separately 

 the roots and the branches. 



a. The roots of origin. — The earliest stage in the development of the 

 fifth nerve that we propose to treat of in the present paper is that 

 represented in fig. 1, taken from a transverse section through the 

 hind-brain of an embryo at a stage intermediate between i and k, the 

 plane of section passing on the left side through the roots of the 

 trigeminal (v). 



As shown in the figure, the roof of the hind-brain is very thin, and 

 passes rather abruptly into the much thickened sides. To the top of 

 the thickened portion the fifth nerve is attached by a long slender 

 root (fig. 1, v a). This root passes down alongside the brain, but not 

 in actual connection with it, widening considerably as it does so. Its 

 inner border follows the curvature of the brain until reaching a point 

 about half way down the sides of the neural canal ; it then turns 

 suddenly outwards, leaving the brain altogether, and forming, as it 

 does so, a very conspicuous blunt projection (fig. 1, v /3), which is in 

 rather closer proximity to the brain than is the part of the nerve 

 immediately above it. The trunk of the nerve then passes outwards 

 and downwards, lying just beneath the superficial epiblast, between it 

 and the outer wall of the second or mandibular head cavity (fig. 1, 2). 

 The whole of the nerve, including its root, consist of closely-packed 

 spherical or polygonal cells, which, like all nerve cells in the early 

 stages, stain very deeply with osmic acid, and differ materially in 

 appearance from the much less closely arranged mesoblast cells. 



By stage k the root of the fifth nerve has undergone very remarkable 



